Try the following:
Move the folder ~/Library/Mail onto your desktop. Do the same with the file ~/Library/Preferences/com.apple.mail.plist.
Now launch Mail. If it starts okay, that means there's something awry in either your mail items, or your preferences file.
If all of your mail is saved on the server (rather than the "On My Mac" section), you can simply set up your mail account again and all will be well.
If you have some mail in an "On My Mac" folder, you can re-import them by selecting File:Import, and then choosing "Mail on Mac OS X" from the list:
Then navigate to the Mail folder on your desktop, then the Mailboxes subfolder, and choose a mailbox.