So what did I do about it? And what can you do about it if a similar thing happens to you?
First, I took a couple of days to just rest. I did whatever I felt like doing. If it was crying, fine. If it was getting into bed and alternating between napping and staring at the wall, fine. I started with the easy ones like those and ended up with more enjoyable ones, like reading The Road, by Cormac McCarthy (because although it's grim, it gave me perspective on how my life wasn't really so bad), and listening to Maria Bamford's new comedy album, which is GREAT, and perfect for anyone who occasionally goes to the dark side in their thinking. I was still depressed at this point, but I started to feel it lifting slightly.
Then I listed all the things I thought I SHOULD get done, and narrowed the list down to only the things I HAD TO get done, with completion dates. I figured nothing was going to be much fun for a while, so I might as well just methodically go through my work step by step and stop having to worry about those obligations. That helped. I get a lot of satisfaction from checking items off a list.
Still in a funk, I added in exercise: taking long walks in the mornings, before it got too hot and sticky outside. I also fixed my nutrition by adding in more fruits and vegetables and reducing junky, fatty, and sugary foods (not eliminating, mind you, but cutting down). That's when I started to have longer stretches of feeling better, especially for the few hours after the walks. Those walks also gave me good opportunities to think through my problems and decide how I wanted to handle them. It's much easier to think when I'm away from my usual environment, which is full of chores and time-wasters like Facebook games. Which I had been playing too much in an effort to just not think about it for a while. That escape was probably helpful at first, but after a while it's time to stop hiding and make some changes.
This whole issue was why I delayed my Victory Mohawk. I wanted to be able to feel sincerely victorious for those pictures! And I did.
Now I am back to my usual level of cheer, with some new insights, and my energy level is back up. So! If you have occasional funks or depressions, following a similar process may be helpful. If you have serious depression frequently, add a call to your doctor ASAP, because frequent, intense depression is an illness that needs treatment just like any other serious illness.