How do I regain my momentum in research after a long period of slacking off?
This may sound stupid, but a good place to start when confronted with a seemingly unsolvable problem is just to start doing something. There are always some simple experiment/simulation one can run, some related papers one can read, or some results that can be written up. The point is to get back to making consistent and incremental progress, no matter how trivial it may seem or how many mistakes you make. Once you get the ball rolling everything else should follow, given your background.

Don't worry too much about coming up with research questions. Good research problems are not dreamed up in a vacuum. You have to immerse yourself in the literature and your research problem to see the gaps that others have overlooked or come up with new methods to tackle old problems. So when you are still trying to get back to speed don't expect brilliant ideas to just pop into your head.

Another thing you should do is to set up weekly meetings with your advisor. Or if your advisor is not available, then write up regular progress report, both for yourself and send it to your advisor. The fact that you can slack off for years and get away with it means that there is a lack of structure and discipline.