Do you have any ideas about what happened?
I'll tentatively suggest treating for anxiety.
I went to a very a difficult school and suddenly found that the single good quality I knew I had (being smart) I didn't really have at all. Every attempt to think hard became a reminder that I am a failure in this regard (and therefore have no demonstrable value as a human being etc. etc.). The existence of due dates forced me to think hard pretty often anyway until I graduated, but since then it just doesn't happen. Apparently this effect is pretty common for alums of [college], but usually it wears off after a year or so and it's lasted longer than that ...
One of the big variations I see between people is the amount of energy they habitually put into thinking, and I haven't seen this discussed anywhere.
General advice about improving health and lowering intellectual friction would seem to help increase the ability to think, and ideas like "take five minutes to consider the problem" adds impetus, but I'm not sure what the general difference is between me and most people, or Yvain and me.
Intellectual drive isn't an unalloyed good-- cranks have high drive combined with low self-editinig, and some types of depression include a compulsion to think about topics that cause misery and/or inertia. Part (all?) of the value of meditation is getting some time off from thinking. Still, increasing intellectual drive would probably be a good thing for a lot of people.
Has anyone found that rationality training or anything else increases the default desire to think?