You do not have to buy a domain to be able to keep your e-mail.
My recommendation to prevent this from happening in the future is to
never use the e-mail service provided by your ISP. In general, your ISP does not care about your information -- and they will say it is your responsibility to save what you want to keep. They store all that information on their servers, and if it gets lost, it's gone.
What you need is two things: (1) an extra layer and (2) a form of backup.
What I'm calling an
extra layer is meant to free you from the confines of your present ISP (there's a word for this, I want to say "transparency," maybe that's it, but I can't remember!). If you move, you will probably need a new ISP. If your current ISP goes out of business, you'll need a new one. If your want to change for better prices, again, you'll be changing ISPs. In any of these situations you may potentially lose all your e-mail. Instead, get a free online web-based e-mail service (I recommend Gmail, see below). Not only can you check your mail from anywhere in the world, but if you change ISPs (or don't even have one) you will not lose your e-mail.
Next, you need a way to
back it up. This is so that if the web-based service has trouble with their servers and lose the data, or if they go out of business (not likely, but possible), or stop offering the service free, then you can still get to your e-mail. Again, this can be free. All you need is something like
Thunderbird (a free e-mail client, of which there are many) or you could use Outlook. I got a free copy of Entourage (Outlook for Mac, as much as I can't stand Microsoft) and this is what I use.
After you get both of these, you can hook them together (I highly recommend
Gmail for this, they provide the technical options to make this possible:
POP3 or
IMAP; the two technologies are different with different benefits, but it would be hard to explain here). Your e-mail client can be linked to your web-based e-mail. If you do this, all your e-mail will be kept in two places: (1) on the servers of those running your free e-mail service and (2) on your own computer. You will be able to read your e-mail from your own computer and also from the web. You will also be able to send e-mail from both places.
Linking them together is not difficult, but I cannot put instructions here because they would have to be tailored depending on which web-based e-mail service you use and which e-mail client you have on your computer.
If you do this, then if you change ISPs you'll still have your e-mail (on your own computer). If your computer dies, you'll still have your e-mail (on someone else's computer). This is much safer than using your ISP's e-mail service and can be done for free.
I know this can't help you get your e-mail back this time, but I hope this helps you in the future.