Any1 know if u can lvl up if uve killed everything in the game?

Restart in a higher difficulty level, there's more experience there.

Are you playing in single player or multiplayer? Original Diablo or Hellfire?

If you're playing original Diablo single player there's not much to do. You can start over (just choose the character and select new game), but unfortunately you will have to begin in church (which means near no experience and crap items). IMO you're better off creating a multiplayer character. If you don't want to play on battle.net you can choose direct connection or ipx and play by yourself.

You can also try getting ahold of the Hellfire expansion, but it's quite hard to get. Unlike original Diablo, you don't have to begin in church when creating a new game, as long as you're high enough in level.