Lethargy.
It's a powerful thing, isn't it? I was like that in early November - any excuse not to go out to my garage and row. Too tired, too cold, monitor not working, any excuse, you name it, I used it. What got me going again was a combination of things:
1. The weather preventing me from cycling, so I felt I had to do some exercise, and
2. This stupid 100,000 metre challenge thing. It's so daft, you're not competing against anybody, it doesn't matter if you fail, but it just motivates me enough to keep going.
Don't know if that helps you at all, probably not as everyone is different, thankfully.
Anyway I've just been out and done 13,000m, so now I'm on 90,000, which means that I should hit the target with my next session.