|Are you sure they've actually been deleted ? |
Most email programs that you install on a computer are set up by default such that the email goes to a different email mailbox, e.g. Trash - the first time you delete it - it's still there until you go to that other mailbox and delete it.
If it's still there in the other mailbox, you can Restore it to whatever email mailbox it was originally in.
If it's a web based email program, it's impossible to restore your deleted email unless it has that feature and you didn't actually delete it.
If it's an email program that is installed on your computer and you actually did delete the email, you could try using an un-delete or data recovery program, but the longer it's been since it's been deleted, the less likely it is that will be successful.
OR - if the deleted email was on your hard drive, if you use System Restore to load a previous restore point that has a date / time before you deleted the email, that MIGHT restore your deleted email.