I use Windows Live Mail on Windows7.
I use the Save facility to keep special important mails in folders in My Documents as .eml.
Their icons changed to a blank square instead of a little envelope so I knew something was wrong.
I could not open any, and suggestions on this site did not work.
I went to Control Panel to Programs, scrolled to Windows Essentials, right-clicked, chose Change, Clicked on Repair.
I was told it would take some minutes, and when I returned it said All Repaired.
Tried opening one and it was open immediately.
Several days later after shutting down each night everything is still OK. As simple as that !!