Reset PRAM in MacMini 3,1 (Early 2009) and now it won't boot at all, but powers on and chimes
I fixed it. Hail Mary solution was to unscrew and take the cover off the SuperDrive, manually remove Leopard install disk #2, lay install Leopard disk #1 in there (much easier than expected), then power on with C key. (I think it was the second try, I believe I hit and held C before hitting power.) Then I got the Apple logo and was asked if I wanted to install Leopard. I erased the drive and installed Leopard. Booted perfectly after install. Now I'm installing Snow Leopard (from upgrade DVD, which I still had), and afterward I'll install El Capitan by download (the OS I had before). 

If that didn't work, I was going to buy a new Mini tomorrow morning. It was the last thing I could think of.

So apparently the PRAM reset screwed up my HD real bad. Before the PRAM reset I had very recently run disk repair from DVD and from the boot HD and repaired permissions in Terminal (like within 12 hours before).

If anyone cares to advise, what's the best way to install a Time Machine backup from my external HD after I install El Capitan? Migration Asst? Recovery Mode? Thanks.