I'd say it's likely dead. It happens, eventually they stop functioning or slowly begin having more and more issue reading discs until they stop all together. Even some poor quality new units have issues like this. Back when dvds first came out, I got a brand new player (for the tv, not the pc) and half the brand new discs out of the package wouldn't play. Or would play partway through then have errors. After returning a couple discs assuming they were defective since the player was new, realized they played just fine in a better player.
Considering dvd burners are so cheap now (usually under $20) I'd suggest just replacing it.