Try pulling your bobbin out of the case - brushing out anything that may or may not be there.
Then re-install, making sure the thread is in the right direction.
Replace the needle and re-thread, making sure its in the right direction.
When starting out stitching, hold the "tails" for the first few stitches.
If that doesn't work, try a new bobbin.
Sometimes machines just get plain ol' persnickety.
P.S. This has happened to all of us at one time or another.
Okay.. at least it's happened to me!