Bitter roasted beets
Roasting caramelizes the natural sugars in vegetables and, eventually, burns them.  Roasted anything that turns out bitter is almost always the symptom of overcooking.  Sometimes it also means that your ingredients were spoiled or "off" in the first place.

As for saving them - there's not much you can do in either case, other than trying to mask it with some other flavours.  Beetroot pairs well with cranberry and other tart fruits, which would probably mask the bitter taste; cream, sweetened yogurt or sour cream would also be a good bet.  Otherwise, I can't really think of anything else to do with them; overcooked food usually ends up in the trash for me.