Thank you for replying.
Shortly after I bought the car, I found a shop sales receipt in the glovebox, for replacing an ignition coil. Under "other recommendations" on the receipt, the mechanic had placed "clutch slightly worn". That was around 30,000 miles ago.
I haven't driven enough manual cars to know how to tell whether it was degrading or not, but it did make the odd slap noise now and again when pushing the pedal in. The total meltdown to undriveability last night was more or less instant though. It drove normally on the way to work, but when I came out to leave work, it wouldn't go into gear.
Clutch was my logical first assessment as well; I'll contact you tomorrow. Thank you for your help.
EDIT - I just remembered that yesterday morning, driving during heavy rain, I drove through a 3-5 inch deep strip of water that I didn't see along the right side of the road. I didn't feel any ill effects from it afterwards, aside from steering being a bit sluggish for the next mile or two. Could this somehow have contributed to a hydraulic issue in the clutch?