Saved e-mails are gone

Hi,  I had a lot of saved e-mails from 2-4 years ago that have just disappeared.  My inbox is never full.  I have plenty of space.  I had these saved as they were reciepts from a truck rebuild we had to do.  I did not print all of the correspondance, as I was saving them in e-mail form.  I need those reciepts and correspondances now, went to look for them this morning, and they were gone.  Can I get them back?  Why would they just disappear?