Possibilities: the tension is set wrong; the bobbin is in backwards - check how the thread is coming off the bobbin, which way bobbin is supposed to spin; bobbin wound incorrectly; needle is nicked, bent, damaged; machine threaded wrong - sometimes you don't have it quite right in the uptake.
Clean out everything very carefully, brush out lint! Check needle (damage can be very slight, use magnifying glass, or just replace needle with brand-new one). Then wind a new bobbin, making sure thread goes on evenly. Put bobbin in, noting directions carefully, ensure when you pull on thread, it pulls smoothly, evenly. Rethread spool & ensure it's through the uptake properly. Check to ensure needle threaded in the right direction (is it supposed to go front to back, or right to left, etc). Bring up bobbin thread, one turn by hand of the flywheel. Then do a couple stitches on the fabric, turning the flywheel by hand to observe every thing.
It's more commonly something rather simple, even stupid. But if you find something is seriously out of alignment, or cannot correct it, call your sewing machine company's help line, or take it in. If the sewing machine has been sitting unused for a long time, it may need oiling & a "tune-up."