Just download the game a few hours ago and played for a bit (it launched and updated fine). I exited the game and it now refuses to start. When I try to launch the game from steam, it just flashes for a second and nothing happens.
I have the newest version of java, my drivers are up to date, I tried restarting the computer and restarting steam, even re-downloaded the game, but nothing is working.
I'm on windows 7 64bit and have a ATI mobility Radeon HD 5730.
Check the Spiral Knights folder and see if it has any hs_errPID####.log files. If there is, open it in notepad and check the fault type and model. If there isn't any, try disabling the Steam overlay and see if it'll show an error.