I think there's been some research that professional/thinking jobs can be as taxing on someone as hard labor like construction. Your brain needs time to relax, decompress, and wander. You have a stable job and free time after work to do whatever you want. This is part of being human - your hierarchy of needs is mostly complete, I assume.
Set all of those things aside for now (book, duolingo, web-dev, etc.) Just chill. Don't treat hobbies or recreational activities like work. The interest in them will come back to you naturally. If it doesn't then were you really that interested? Or was there some observation of others falsely creating a desire?
If one of your interests is career related (web-dev), seek out time during your work day to further your career. Sneak it in if your employer won't allow it.