Friendless..seeking good advice?

I am in my twenties, considered nice, pretty, active, friendly and will be returning to school starting a second career. I should feel on top of the world, expect for the fact I am totally friendless. Totally. No friends, no family. Had a boyfriend, but he stopped calling me w/o a reason. It gets hard knowing I have no one to call and talk to or just hang with. I am friendly to others, but it seems like everyone is involved in their own lives already. I find it harder to be happy and nice, like I dont have any energy left and no one loves me. Please help, if anyone has advice or anything...Thank you!