After deleting your old main, you need to create a new one in order to specific a character as a main (characters created after you designated your main are alts, and can't be changed from that.)
If you severely regret deleting your old main (you had too many drinks, you lost a bet [welching out is stupid, btw], you were just derping around), try contacting whoever is in charge of that stuff.
It's listed somewhere in the forums.
