Over-tightened nut, bolt broken?
Remove the bolts and inspect the threads of both the bolt and the tapped (threaded) hole in the frame.

You have three possibilities here:
Either A) the frame threads are damaged, B) the bolt threads are damaged, or C) both threads are damaged.

If A) or C), you're unfortunately out of luck without drilling and tapping a new hole as Tyler has said (this won't work if the frame is plastic with threaded inserts). This is the most likely because your bolts are probably steel and your frame is probably aluminum, or plastic with inserts.

If B), take the bolts to a hardware store (Ace Hardware usually has a great selection of individual bolts and are usually more helpful, but a place like Home Depot or Lowe's will also work) and find new ones that match. Then take care not to over-tighten them!

General rule: if there isn't a torque specification on a bolt, run it down "snug" (until you start to feel resistance, then give it another quarter to half turn). "Snug" is subjective but it should get you pretty close. And it'll help when assembling cheaply-made parts.