Well, it can't hurt to install everything again.

There should be a disc for Cinematics, then an Install Disc, then a Play Disc, which is all for Diablo II (Not Lord of Destruction). The Play Disc should have the Diablo II CD-Key on it, which you must register.

Next, you must install the Diablo II: Expansion Set (Lord of Destruction). This disc should have another Diablo II: Lord of Destruction CD-Key on it, which again, you must register.

Then you should be fine.

Good luck!