Wife's 97 Subaru has been limping along for years, but finally died the other day and refused to start at all.
Pulled the codes from the OBDII. Two of them were ongoing codes I keep putting off investigating, because its ran...until that day. The third code was P0325 - for the knock sensor.
Apparently if the knock sensor gets corroded enough (and it looked pretty bad) it'll progress from hard starting to a straight up no-start.
Ordered a Chinese import part* for $7.99 - boom, fixed!
*Note: normally I wouldn't go this route, but I wasn't 100% sure this was the cause, and while the cheapo sensors don't last terribly long, if they average a year it would take me 10+ sensors before getting the Subaru approved part would be cheaper (even through non-dealer parts stores).