Fortunately I worked it out before completely losing it.
After finishing a rag quilt for my nephew, I thoroughly cleaned my machine to get rid of the residual lint.
Last night, I started a new project and wasted quite a bit of thread when the fabric was not advancing. I disassembled the machine again, checked to make sure that the feed dogs were not blocked and could raise and lower correctly. Still nothing, more thread wasted.
Then I happened to look at the stitch length indicator. Yep - set to a big fat 0 as the last stitches done were to lock the threads.
So, fixed that and created a very impressive birds nest. Disentangled the thread from the bobbin mechanism and started over. Fortunately before starting the next bird's nest I asked myself why the tension would be so terrible - hmmm - could it be that the machine is not correctly threaded AT ALL.
The nearest I can figure is that when I finished winding bobbins I laid the thread over the top of the machine and when I sat down to get started again, simply threaded the needle, assuming I'd run through the other important contact points.
All's well that ends well. I did sew 10 strip sets last night for the 9-patch challenge that I'm participating in. And I was able to solve the problem myself.
Cheers, K