In multiple Star Trek: The Next Generation episodes, Counselor Troi has a headache or "feels tired", goes to her quarters to lie down, and it later transpires that something relevant to the plot has happened to her.
How many episodes incorporate this subplot? I'm sure I've seen it more than once.
I went through Troi's Memory Alpha biography looking for matching patterns, and then combed the scripts of the episodes I judged most likely to contain this plot point.
As Richard notes, "The Survivors" is the best example, though worryingly it does not appear on the wiki page. I'm not quite sure what to make of that.
"The Loss" actually is an example, because Troi does not realize that she's lost her powers for quite some time:
[Sickbay]
CRUSHER: Yes, Deanna?
TROI [OC]: Beverly[Troi's office]
TROI: I'm feeling very dizzy.
CRUSHER [OC]: Did you hit your head?
TROI: I'm not sure. I'm not sure what happened.
Jump ahead a few scenes:
[Troi's office]
CRUSHER: I got here as quickly as I could. I've got a Sickbay filled with headaches. How are you?
TROI: Well, when I first called you I was feeling intense pain. Now it's gone, but I'm feeling a bit foggy.
CRUSHER: I'd like to take you to Sickbay and run an inner nuncial series. Can you walk?
RIKER [OC]: All senior staff report to the Observation Lounge.
CRUSHER: Are you up to that?
TROI: I think so.
(then she gets a look of horror on her face)
CRUSHER: What is it?
TROI: Nothing. Nothing, I'm fine.
While she could be (read: probably is) in denial at this point, it does fit the overall pattern quite well.
After she does realize that she's lost her powers, she seems to believe it is not a serious problem and will go away on its own:
CRUSHER: No indication of a concussion or a blow to the head. Just a pattern of unresponsive neural cells throughout the cerebellum and cerebral cortex.
TROI: Unresponsive?
CRUSHER: There's brain damage. How serious, I can't tell yet.
TROI: But I feel perfectly fine otherwise.
RIKER: Can you treat it?
CRUSHER: I'm not sure. I have to review the database on Betazoid neurophysiology.
TROI: The Betazoid brain has a remarkable ability to heal itself. This condition could just reverse itself time.
CRUSHER: It might. But you are half human. That changes the map a little. I don't want to offer you any false hopes.
TROI: It's just hope, Beverly. Not false hope.
RIKER: What would cause something like this? Because it started at the same time we encountered the anomaly.
CRUSHER: There may be a connection. I don't know. If you were anyone else, you know the first thing I'd do? I'd send you to Counsellor Troi.
TROI: Well, then I have an advantage, don't I? I see her quite often.
CRUSHER: Deanna, it's no different than one of us suddenly going blind.
TROI: You don't have to tell me, Beverly. I understand the psychology.
CRUSHER: You may understand it but you've never had to live with it.
TROI: I may be perfectly fine by tomorrow.
CRUSHER: And you may not. Now, I'll do my homework. I'll see what I can do to regenerate those cells. In the meantime, I want you to talk to someone. There are several people on board who have degrees in psychology, who are qualified therapists.
TROI: Okay, fine, if I need to. Right now, I just want to go back to work.
RIKER: Deanna.
TROI: If there are no medical objections?
CRUSHER: No medical objections.
TROI: Thank you, Beverly. Will.
"Night Terrors" is loosely similar, because Troi initially believes her dreams are "just dreams," but I don't think it quite counts.
She behaves oddly without apparently realizing it in "Man of the People," but isn't relieved of duty until she is already comatose. At one point she does randomly cancel some appointments, one of which incidentally has a rather interesting name:
(Deanna checks herself in a mirror. Is that a wrinkle? )
TROI: Computer today's appointment calendar.
COMPUTER: Oh nine hundred hours, counselling session with Ensign Janeway. Ten hundred hours, counselling session with
TROI: Cancel them. Cancel everything till after lunch.
COMPUTER: Acknowledged.
But this doesn't really fit the pattern all that well.
Finally, in a rather interesting reversal, Star Trek: Nemesis actually has her figure out that something is wrong before she asks to be relieved of duty:
DEANNA: Shinzon's Viceroy seems to have the ability to reach into my thoughts. I've become a liability... I request to be relieved of my duties.
PICARD: Permission denied. If you can possibly endure any more of these assaults. I need you at my side. Now more than ever I...
This is not from the final version of the script, so it's probably wrong in some way, or the scene may have been cut completely.
I'm fairly sure you're describing a single episode; TNG: The Survivors.
TROI: I'm sorry... I don't feel well. I would like to go to my quarters...
PICARD: By all means.
BEVERLY: Is there anything I can do?
Troi shakes her head and leaves the room without another word. Picard watches her go, concerned. The others too.
There are other episodes that seem to fit a similar pattern (TNG: The Loss for example where she goes to her office and collapses) but no others that directly match the trope:
8B INT. TROI'S OFFICE
Troi tries to reach a couch, doesn't get that far. She clutches her head, knees buckling, as she crumples to the floor, unconscious.
and in TNG: Encounter at Farpoint, she seems to be suffering head pain as a result of a having encountered a surprisingly powerful mind:
Data trails off his words, finally becoming aware of the annoyance registering on Picard's face. Troi cannot keep back the smile now ... then suddenly her face is contorted in pain.
TROI: Captain ... I'm sensing a ... a powerful mind ...
There was one point in Clues when Troi was feeling dizzy (later revealed to be because an alien had "possessed" her), and she says "I think I'd better go to my quarters." Picard suggests she go to SickBay, but she dismisses it as not serious.
