Did you perhaps update any software that could have caused this? Like your graphics drivers? In any case, try if there is an update for your driver, that might help.
You could also try running it without mods&the mod loader, just the vanilla install. Mods _shouldn't_ be able to cause the game to crash like this but it can happen.
nothing. I tried to use vanilla besiege, I updated graphic drivers, and nothing. the screen turns black as I click on the sandbox or a level, as every time, but it appear a window that says that besiege.exe has stopped working. I guess that the output log isn't helping
Bio-4 It's not helping that much, no. At least I can't find a lot of useful things in there.
Is there perhaps a crash log available? That would be a folder named after the date of the crash, inside your Besiege folder, alongside Besiege_Data and Besiege.exe. It might (might!) contain more useful information if it was created.
I think in Steam you can verify the local files and it will download and repair any that are corrupted. worst case scenario....make a backup copy of all your saved machines and reinstall. Besiege barely takes any time to reinstall if you have basic broadband.