I find that sometimes there is just no motivation, there's even a big reluctance. It's very weird how or why this happens now and then. Maybe it is a natural protection mechanism kicking in, or maybe it is just a chemical imbalance of hormones involved in the reward system.

'Just do it' (tm) does not work in those cases.

After a while i just google 'how to get out of a rut' and that works after some undefined amount of wasted time.

Another trick to fall back onto, is changing jobs.