I've had this happen two times to me. One was on a Brother and the cause was I was threading the needle from left to right instead of right to left. The second time it happened it was on a Janome and the needle bar slipped a fraction of an inch. The set screw that hold the needle bar worked loose letting the shaft drop. Open the left end of the machine so you can see the needle shaft and using a strong light check the shaft and if you can see an indication line where bar was held in the shaft then gently loosen the screw and push it up to the mark and tighten the screw to hold it in place. It will probably be a set screw, using a small allen (hex key) wrench. That saved me a small fortune in a service/repair bill. I hope you find a simple solution which it usually is. Be sure and post how the problem was fixed. I'm keeping a log of fixes so others can reference it when needed.