if it is a redeamable slot or purchased slot you wont be able to use them when the store is down. this is a known issue and has popped up through out several of the different threads here on the forums. they are working as quickly as they can to get the issue fixed.
It appeared to be fixed last time I switched toons.
Paragon City Search And Rescue
The Mentor Project
i have just deleted a toon and now the slot says unavailable... is that happening anywhere else too? i want to make a new toon, but cant
-matt