I got up this morning and was a functional adult again. I did some home maintenance projects (including one I have been putting off for YEARS), did some boat maintenance, and generally coped with life. The contrast between the two days was scary, and I don't know what caused it.
One odd thing that has come out of this is that I have realized that the opposite of depression is gumption; Monday I had one, today I had the other. And I have no idea why. Depression is often linked to despair, but it is a haphazard relationship; my despair is chronic and constant (and, being based in intellectual reality, incurable), but depression seldom actually bites me, certainly not as solidly as it did yesterday.
I am somewhat amused to realize that worst depression crash I have had in a long time laid me low when I was trying to do something recreational and pleasant. I have coping skills that get me through all of the drudgery of my life, but it seems that they don't apply for elective activities that I actually WANT to do. We shall see.