Odo gets knocked out a couple of time, firstly (As @KyleJones has pointed out) in DS9: Vortex by what appears to be a blow to the shoulder. At this point he's in changeling form.
But as they pass through the tunnel, there's a direct hit and rocks come sliding down on top of Odo...
CRODEN : Odo!
He leans down and examines Odo, who is unconscious...
It's not especially clear what caused his injury but from reading the script, it seems likely that the writer originally intended there to be a lot more rocks!
He also gets punched out (by O'Brien) in DS9 : The Assignment but this is whilst in human (non-changeling) form