1. BREATHE
First re-thread both the top and the bobbin threads to make sure everything is right.
Are you using the same thread in both the top and the bottom? In most cases it's best to match the thread in both.
2. If you have changed your bobbin tension, start by adjusting that so that when the bobbin and case together are picked up by the thread, the thread will just ALMOST feed itself out. Mine works best if I can just barely pick it up without the thread feeding out.
3. Once that is correct, rethread the top and start testing/adjusting until the top and bottom tensions are "balanced".
Sounds simple, but sometimes it takes a bit of fiddling around to get it just right. And, expect to adjust it when you change types of thread.
Hope this helps a bit.