Something like this happened to my FW recently. Maybe a drop of oil...and that 'finger" that is part of the post that holds the bobbin case- when you took off the throat plate, that part will rotate. I found my little piece of thread by rotating that ..hemostat/tweezers and turn the hand wheel back and forth gently while holding the thread taut.
When you get it out, that "finger" has to go back into the space on the underside of the throat plate. (ask me how i know!) Deep breath...and maybe turn the machine so you can really see under there. Sewing machines are pretty much the same. Any Singer dealer can service your FW...I take mine to my Bernina dealer! Hang in there!