First step: BACK UP. Make MULTIPLE COPIES.

When was the file dated?

If it was from say 2009 or 2010 then it's safe to assume it's a bitcoin one.

If it was 2011 it could be Litecoin, and if from 2012 it could be peercoin, or some other crypto from 2012.

First try bitcoin, and see if it comes up with anything. You could be in for a very pleasant surprise!