Several friends of mine have all had the same catastrophe lately. I won't bother telling you what it is since you'll be able to figure it out from the following advice…
Do not store your e-mail on someone else's computer. I know it's sometimes easy to leave it all in the box on AOL or Hotmail or GMail or wherever it arrives. Do not leave it there. Use an offline e-mail program and download it to your computer. Read and answer it offline and then send your replies from there.
The program I use (I'm a P.C. guy) is called Forte Agent and it's very good. Most of you would be better served by Mozilla Thunderbird, which is a terrific program and it's free. The most popular e-mail programs seem to be Eudora Pro and Microsoft Outlook or Outlook Express. I tried but didn't like Eudora and I don't see what it does that Thunderbird doesn't do better and without cost. The Outlook programs were okay but I'm wary of having my ten tons of e-mail so integrated with my massive calendar and gargantuan contact lists and such.
But whatever you use, use something, for God's sake, to download your e-mail to your computer and then you must also maintain backups of it all. Because if you leave your past e-mails (incoming and/or outgoing) on some service, you will go there one day and find they are gone. And no one at that service will be able to help you get them back.