Sounds like you've swapped to a resolution your monitor doesn't support. Don't worry, we added launch parameters in Patch 1.3 -- so first off, make sure your game is fully updated.
- In the Steam client, navigate to the Games tab
- Right click on "Castle Crashers" from the game list, then select the "Properties" menu option
- Under the "General" tab, click the "Set Launch Options" button
- Clear away that silly warning message!! Or... wait, you should probably pay attention to that. It could really mess up your game. O_O
- Copy-paste the following into the box:
This will force the game to launch in windowed mode.
Set the correct resolution for your monitor in-game, under Help and Options > Game Settings > Resolution.
You can also swap to Fullscreen mode here too, but be sure to go back and delete:
from the launch options box later, or you'll have to change your settings every time you boot.