I have some liquid mead yeast that has expired a couple years ago, according to the date on it. It has always been kept in the refrigerator. Could it be revived and used or do I need to buy some new yeast?
A couple years is a LONG time :) I would make a starter with the yeast, if the starter takes off then you're all set. If it doesn't get going, get some new yeast.
These are some good instructions for making a starter