Ran the battery flat.
I rode it "the long way around" to work today. "Work" is in the Leipzig Opera House. As I was taking off my helmet, the Opera Director came past on his way to work. He greeted me, and commented on the bike and so on. As a result, I wasn't concentrating when I turned the key to engage the parking lock. I reckon I turned it all the way to "parking lights".
When I tried to start it to go home after work, nothing.
Fortunately:
There are some tools under the seat. This occurred to me in time to stop me from just going home in a huff, and I took the battery out.
I was all set to take the battery home on the tram, carrying the helmet and far too warmly dressed. Then it occurred to me that we (the sound and video department of the Opera House), by lucky coincidence related to a production that is running at the moment, have a CTEK battery charger set up ready to go in our storeroom. And I could leave the helmet and the excess clothing in my locker at work. So I went back in, plugged the battery onto the charger, and left the excess baggage in my locker.
Hopefully, when I go back in tomorrow around midday after the forecast rain has stopped, I'll be able to put the battery back in and ride the bike home. And hopefully, the battery is not defunct.
Also, I noticed when taking the battery out that the - terminal wasn't done up particularly tightly. The bonus would be that that is the cause of the starting problems it has been having. We'll see.
More likely, I'll have to get a new battery, I reckon.