I also lost the private key for access to mew. Unfortunately I did not make a backup because I only store in one place only. As a result I lost some tokens.
This is a lesson for us to be more careful to keep the private key, because the account that access using private key can not be opened again if the private key is lost, unlike the account that access using password, if lost or forgot can use email to open it.
So let it go that already lost even though it is very sad because there is no way to open and return the lost coin.
All you can do is create a new account with a new private key, save it well if you need to back up as much as possible to anticipate.