I went out and punched the button for the garage door opener this morning. The door went up a couple of inches, then immediately turned around and went back down. After several button pushes, I determined that this trick wasn't going to work, so I pulled the door release and went to lift the door by myself.
Oof! The door was incredibly heavy. I got it up all the way. daisy_knotwise was standing outside, because the original plan had been to pull out the ladder so I could replace some light bulbs, but the garage door problem was now urgent, since both cars were in the garage. Gretchen ran in and got a step stool so she could hold the door open and I failed miserably at relatching the door release so that it would stay open.
"Hang on," I said. "I'll pull my car out, then I'll hold the door while you pull your car out." And we did and I carefully lowered the door back into the closed position and called the service company with a sticker on the back of the door that we'd used for the most recent garage door opener installation.
They told me to look at the springs. Given that one of them was in two pieces, that pretty much explained everything. Happily, they were able to send someone over to replace the springs today, so the garage door is working again.
But that thing is still heavy!