You probably knocked it out of time. There are two push buttons on the base of the machine. One of them is for resetting the timing. Press the button down and rotate the hand wheel until the button (locks-in) to a groove continue to rotate hand wheel until you hear a click. That click is the clutch re engaging, after doing this you should be able to sew just fine.
Chris
glfi.com
Shorten your stitch in reverse the machine can't do a stitch that long in reverse. Also google your make and model machine and the word Manual so you can properly learn to use the machine safely.
Cheers,
Kelly
San Diego, CA
Embroidery Services
Tilt the machine up. There's a screw holding the hook in. Loosen, remove base plates, lift the hook out and replace. Don't move any thing else or you'll have to reset timing.
Remove the belt. Check that the bobbin case is installed correctly. Hold the button down while turning the wheel. Also check the lock screw on the shaft. It can get loose, or too tight.