The sheetrock in the ceiling over my dining room has developed a crack. The
crack has begun 'running' across the ceiling.

If the crack is not following a seam or joint, you may have a nasty structural
problem to deal with. If your house is a ranch type on a slab with rather long
ceiling spans there could be
enough movement to cause cracking. A good architect or structural engineer
would be familiar with these problems. It would be prudent to get the best
advice you can to get a permanent fix. Good luck.

Joe