Mouse is rotated by 90 degrees

Check the mouse settings. There are some mouse drivers that let you change the direction of movement (or rotate the direction of movement). I think this is for use with trackballs.

There is also a program, Sakasa Mouse, that will rotate the mouse movement. The program is intended as a joke so check if someone installed it to mess with you. You shouldn't install it to fix the problem though, because as GwenKillerby states below, it's full of malware.

Barring that, you could turn your monitor 90 degrees the opposite direction and use the computer while lying on your side. ;-)

I had that. Happened after the screen was rotated, and then rotated back to normal. Mouse wouldn't rotate back to normal.

Rotated the screen several times to different degrees of rotation, and then back to normal. Somehow shook it up and it came back to normal.

Don't know how it happened and the fix makes no sense, but it worked. Try it.