Mail vanished into thin air!

[stepht]

I was sending an email when my Mail programme suddenly disappeared from the screen of my ibook OSX 10.4. When I try to re-open Mail now it wants me to re-start a whole new account ie my Mail account of the past 4 years seems to have disappeared...
I have been having some trouble with Safari for the past few months. When I open some pages, they suddenly disappear and I get the message 'Safari has unexpectedly quit.' I wondered if the two 'vanishing' problems are connected. Mainly though, is there any way of retrieving my previous Mail account. (I think it was a POP account, ie everything was on one computer.
Thanks for your help,
Steph