8250 DiskDrive don't work

Igal, have you tried with another disk? What happens when you boot the MSX without a floppy in the drive? What happens when that works, you insert disk (after booting without) and then do "FILES" ? And what happens when you hold [SHIFT] pressed during boot?

Other than that, it could be that the floppy drive itself has died. Being an electro-mechanical device, I'd suspect that first rather than the FDC. Another option (besides many other possibilities) is that system software is in EPROM & bits have started to 'leak'. EPROMs were used in many 8250/55/80 machines.

And indeed check power supply first (+5 and +12V, it's probably safe to ignore the -12V supply)

Oops I missed that...

Btw. could be a simple cabling problem, where the drive is started and then the FDC waits for some signal that never comes. For example I've seen floppy drives that would happily write to a write-protected floppy, because the "write protected!" signal didn't get passed to the FDC correctly. So you might want to measure the cable connections (multimeter, low resistance range, measure from a point on the 8250's mobo -> point on the floppy drive's controller board, so you catch everything in between). Or plug in another 8250 floppy cable & see if that fixes the problem.

Floppy cables are usually reliable enough, but remember there's 4 contacts per signal (mobo pin -> plug, plug -> wire, wire -> plug on the other end, that plug -> floppy drive pin). And at least a dozen signals passing over the same cable.