Mary has been listening to library books while she quilts and sews; some of the older books actually come on cassette tape and our tape players still work. The newer books come on CDs, but it seems the library hasn’t gotten into audio e-reader files yet. She actually prefers tapes, because she can simply stop the tape and restart it from the same place without any further intervention.
In any event, a recent tape stalled about 1/4 of the way through and refused to either rewind or fast forward.
Rather than returning it to the library, which I’m certain all previous borrowers did, I took the cassette apart. This is no big deal, I’ve done it many times before cassettes fell into the dustbin of history.
That made the failure quite obvious:
The bushing around one of the hub openings had completely fractured and come loose, jamming the tape hub in place.
A ring of solvent adhesive around both parts, a few minutes of clamping, and it’s all good again.
Don’t tell the library; they get tetchy about DIY repairs…
The Smell of Molten Projects in the Morning 
For the shells that were actually too busted to repair (or were glued together such that they splintered when separated), it used to be possible to buy repair kits that consisted of a complete shell with pressure pad. Some even came with hubs and leader so you could splice in the tape if need be. Somehow I doubt such repair parts are easy to find these days.
One of Mad Phil’s treasure boxes had a stock of leader and splicing tape for reel-to-reel recordings: that’s even older than cassettes!
I ran it past the local hackerspace and nobody had a use for it…
Heh. I don’t know if I still have my R2R splicing block. It must be 40 years old. About 20 years ago I bought a mildly used Revox 77 (two track!) and got a bunch of (mostly bulk-erased) tapes with it. It’s a lifetime supply, I think. Haven’t fired up the ‘Vox in 10 years, must give it a try.
My last cassette deck (barring the one in Julie’s Ranger) died with a bad shaft-movement detector. Wasn’t worth fixing.