Same boat here :\
I use kuler for work for multiple projects and was foolish enough to not have a backup on another site :[
All my themes are gone and i'm guessing it has something to do with the maintance that was going on recently?
Hope this gets resolved!
UPDATE: Just restarted my browser and logged in to kuler again and ONE of my themes is back? All the rest are still missing though :\
Thanks Dave, mine are suddenly back today, I see there was maintenance and I cleared my web history last night, that must have done it, I hadn't seen the notice,
Try clearing the cache, mine are back.
Cleared my cache and used 2 different browsers on 2 different machines and still nothin' :\