Simple thing to check!
This happened to me earlier - that's how I found this thread, and it turned to to be super simple. I have a Singer Model 750 that has a bobbin winding knob. It's really easy to forget to adjust that knob after winding a bobbin, and if you don't this will happen to your machine. Before you take it to the shop or try to change the timing, check if your knob got knocked out of place!