Thanks to all for the responses!

I don't think it's debris or dirt, or a problem with the valves. I took off the top and bottom plates and had as thorough a look as I could. I always brush my teeth before playing any wind instruments, unless I'm sure that there are no crumbs or such-like in my mouth, and am otherwise careful about keeping them clean and dry.

I wasn't really sure that it was possible for a reed to suddenly go flat like that; I thought maybe somehow the wrong reed was being sounded, so I tested this carefully. Thank you for the confirmation that it is possible, Steve. Fatigue sounds like the most plausible explanation to me.

I agree that the workshop botched the job. It's a pretty good music store, but they're not really experts. I didn't have to pay for the attempt at repair. I'm a bit annoyed that it is now unplayable whereas it was at least somewhat playable before. Since they did botch the job, I'm not that eager to take it back, and it's not worth getting into a fight with the music store. They're friendly, and if they're not really experts, they are at least in town, which is something, these days. I've had my share of experiences with music stores and they are a mixed bag. I think I'll just go and talk it over with them and see if they'll give it another go. At worst, I'll have an unplayable harmonica, which is what I've got now.

The chrome is wearing off the faceplate and, to paraphrase Ira Gershwin and DuBose Hayward, a harmonica is a sometime thing. Thanks again for all the suggestions; one of them will either work out or else maybe I'll be able to fix it myself or replace it when I've got the money and/or place to work on it. I love shop-talk, anyway, though not as much as having functioning instruments.