I'm a PC tech, with very little Mac experience
I have a client that had their email open, things were extremely slow, so she knew something was wrong. She shut everything down - rebooted - and when she re-opened her mail, the inbox was empty.
Since then she has received new messages, and they display fine, but the old messages have all disappeared.

Is there a way to retrieve these messages?
Any idea what could have caused this?

She also complains that often, while in mail, she gets a message "Writing to Disk" - which takes a bit, and she's unable to do anything else.
She says she isn't doing anything that should cause a 'write to disk'.

Thanks in advance for any advice!