This usually happens when something has changed the file permissions of Spiral Knights so that you (as the normal, non-admin, user) no longer have proper permission to launch the game. The most common cause is security software marking the Java game files as a false positive.
Since Steam doesn't always properly uninstall Spiral Knights these re-install instructions should set it back to normal:
1) Right-click Spiral Knights in the Steam Library and choose the "Delete Local Content" option to remove the game.
2) Navigate to your Steam games folder (default C:\Program Files (x86)\Steam\steamapps\common) and delete the entire "Spiral Knights" folder.
3) Go to Spiral Knights in the Steam Store or your Steam Library and install the game again normally.
This usually happens when something has changed the file permissions of Spiral Knights so that you (as the normal, non-admin, user) no longer have proper permission to launch the game. The most common cause is security software marking the Java game files as a false positive.
Since Steam doesn't always properly uninstall Spiral Knights these re-install instructions should set it back to normal:
1) Right-click Spiral Knights in the Steam Library and choose the "Delete Local Content" option to remove the game.
2) Navigate to your Steam games folder (default C:\Program Files (x86)\Steam\steamapps\common) and delete the entire "Spiral Knights" folder.
3) Go to Spiral Knights in the Steam Store or your Steam Library and install the game again normally.