If you get to level 50 you will see that the experience points earned are not counting towards the bar to level up. Mafia Wars 2 currently has a level cap of 50.
We plan on releasing new missions soon, and at that time the cap may be raised.
In the meantime, there are still things for you to do in the game:
Decorate your Home Turf to your liking
Fight in the arenas
Visit the Home Turfs of your friends to help them out
Please be patient with us until new missions and levels are added. Thanks!
--The Mafia Wars 2 Support Team