At&t samsung captivate help?

i have the samsung captivate phone by at&t and its just started screwing up the screen stays horizontal no matter what u do to it i checked the settings and everything nothings wrong with it its worked fine all day i sat it down and picked it up cuz i had a txt and it started doing that it has no water damage and ive only had it a few months how can i fix this? and yes i took out the battery sim card etc multiple times it still doesnt help