It really pays off to crawl through older posts, because I found a reply from a GM that said that restarting steam and/or your computer should make it reappear. It worked for me.
steam logging
Di, 02/28/2017 - 23:29
#1053
Dusk-Sent
Possible Fix
Ok so I have come up with a possible fix for the problem. It worked first try afterwards. Right-Click Spiral Knights on the games list then click Properties. Click on Local Files and then select Verify Integrity of Game Files. Once its all done loading press play and then launch Spiral Knights on the JVM and it should all load for you.
I'm having the same issue as well, I wasplaying the game just fine yesterday, but today it won't let me login through steam