Is it textured or flat? How bad? New house or old? Any other cracks?
If it's not water damage it could just be from the house settling.
That is usually a sign that the ceiling has moved. A builder told use that the glue or screws are slowly coming loose and will eventually need to be secured back up again.
Hi Michelle. One thing that can cause a crack is (if the ceiling is drywall) the tape used between the sheets of drywall may be coming loose. The drywall mud that's used over the tape will crack. If the house's fairly new, settling can cause a shift in the drywall sheets creating a crack. If you've experienced some bad weather, that too can make the home shift. Then there's the scary stuff: water damage and structural damage. If you're not sure what caused it but are pretty sure it's not water damage or structural, patch it with drywall mud and paint to match the rest of the ceiling. If it reappears, call an expert. Good luck!
We bought an old house ( built in 1878) and there were a lot of cracks in the ceiling and walls, my husbands uncle was a builder and he told us to use stiffened lining canvas, you put it up like wallpaper , we used a professional paper hanger to do it because it’s heavy and hard to put up . It spans the cracks and moves because it’s cloth but it won’t tear. You can paint over it and wallpaper over it. We have done it in our living and dining room and 3 bedrooms. 47 years ago and the walls still look great.
Just make sure you don’t get the kind that has a vinyl coating on it, I’m not sure you can wall paper over it. You can find it a good paint store