My Sony DVR-S300 was working just fine. A friend gave me a "DVD" to check out. Turns out it was a homemade Video CD, made in Europe. I put it in tray. Nothing. Now, the player won't even recognize ANY DVD when inserted. Tray just pops open. Setup menu no help. Tried unplugging for long period of time (hours). Nothing. Player unusable. Any hints, clues? Not a DVD pro, more a Mac guy. All help is appreciated. Thanks.

Is it a CD-R? Some players can't read CD-R discs.

How good is this 'friend"? Did he know what was on the VCD? Could it have been some sort of bios flash?

The friend is a friend. As for CD-R, the answer is no. It would not read any DVD (commercial DVD), and does not recognize any DVD in the tray. When the tray is EMPTY, the screen says "CAN NOT PLAY THIS DISC" even though there is no disc. Something whacked this DVD player out. Must've been this bootleg video-CD from Austria (Fellowship of the Ring...had to see if it was really on the disc!). After I inserted that, nothing worked. Is that possible?

Sounds like your DVD player has a sensor that detects whether or not a disc is present and that sensor is faulty. I would try and contact Sony.

they'll probably tell him it needs service since he did a cold boot (unplugged it for more than 30 seconds), but it might be worth a try
1-800-222-7669 then I think it's option 3 for technical assistace then a bunch of other options to get to dvd players
http://www.sel.sony.com/SEL/service/conselec/dvd/