Working on my thesis right now... Can't concentrate at all, I try to focus but constantly fail at it. Any ideas how I can get back into study mode?

Would love to got to the library to write, but due to covid it's obviously closed.

go see the psychiatrist and ask him for medication that helps you to focus. Like Ritalin or Elevanse.

Also eating healthy meditation and sports helps to balance yourself. If you feel really well you'll have much more energy to do great stuff. Even studying.
Ask your parents to hold you accountable. Like "i have to read this chapter by tomorrow" "son did you read said chapter??!!!!"

Play games for five mins, take 10 mins break to work on something small, then go back to playing for five mins. Keep yourself motivated with small things. Create the urgency to get something done so you can focus on what you like to do.

The last line from @NoMad. Create that urgency somehow. Mark goal-deadline events on calender with nearby dates and plan likewise. Make reward systems like if you achieve this, you'll get to play, like that.

Feel you. My trick was to go on a tangent from the thesis in the direction I enjoy. For me it's optimization.
Just checked out to a temp branch and started refactoring shit carelessly. That gets me in the *zone* and I consciously steer myself toward the word document, and let the inertia take me.