Well maybe you accidentally changed your "switch weapons" key to ESC instead of space or whatever you have it as normally. You can access the menu thing that pops up by going to the bottom left corner with the wrench icon and clicking it, clicking "Options." Then go to the "Controls" tab, the fifth from the left...I think, and going into custom controls and finding what your weapon switch key is set to. Also what your open options menu is set to.

The screen freeze may have to do with your internet, or computer. I am not sure which. I get those screen freezes on my internet a lot, but it usually just catches me screen up to what it missed out on.

If the problem keeps happening try to uninstall and reinstall spiral knights. Again I am not a tech expert, apologies if it doesn't get fixed!