According to C-3PO, R2-D2 is in a self-imposed low-powered mode.
From the novelization.
“He’s been locked down in self-imposed low-power mode. He just hasn’t been the same since Master Luke went away.”
C3PO later mentions the condition again.
“Oh, do try and cheer up, Artoo. This enforced immobility is no good for you. Your cognitive circuits will atrophy from lack of use.”
The novelization also describes the condition as being in "minimum maintenance mode"
The squat droid he was addressing sat quietly in minimum maintenance mode, without so much as a single telltale blinking.
While 3PO is certainly fallible, we currently have no other information or quotes which contradict him.