Whenever I launch the game, the screen turns grey and nothing happens. This happened after I turned on borderless fullscreen.

Whenever one has a problem with a game the number one rule here on Steam forums is one posts their specs. I have no way of knowing if you even have the specs to run the game properly.

Without any actual info the best I can offer is for you is to first verify your files and then if that doesn't work, find and delete the config file and see if it will allow the game to come up properly. It should regenerate the config file at start up and this time don't use borderless.

My specs are:
Geforce RTX 3060
AMD Ryzen 5 3500X
16 GB RAM
12 GB VRAM

Also, deleting the config file didn't work. I still get a grey screen on startup.
Steam screenshots look normal, though.

Alright, I managed to reset the settings ingame by using steam-screenshots. (Because these still looked normal)
Now it's working again.

Had the same issue. Had to verify game files and then wait a few minutes for the game to boot.