basically they have same meaing. but if they are in the different secene the means will change for example if u get some problems with other guys then u say 你怎么了？it means what's ur problem? but normaly 你没事把 like if ur friend fall down then u aske he or she 你没事吧 or if some guy wanna cheat some girls and the girls don't interesting about that guy they will say 你没事吧/ haha

你怎么了means what's the matter? 你没事吧means you are worry so you ask whether you are ok or not?

whats wrong and r u ok

when we don't konw what happened to someone and he(she) doesn't looks like before we can say 你怎么了? when we know what happen to somebody we can say你没事吧? and most time the thing happened is not a good thing .

“你怎么了”and "你没事吧" you will say them after something happened, but I think, "你怎么了" : you dont know what happened "你没事吧" you know what happened, and maybe attendant. ahah`~~ it is almost same... I think... even I dont divide them at all in anytime... in my daily life