If my thread breaks (or a bobbin runs out), I go back a few stitches before where it broke, and start again, using some backstitches to make sure the old stitches stay put. I can't imagine having to rip out everything I'd already done, I'd never quilt again!

I also had a lot of problems with the thread breaking, and found that it was a combination of the thread itself,(weak and a bit old) and my needle, it had a sharp bit that was actually cutting the thread.