How do I recover lost emails on my iMac?

On my imac (latest update) ALL of my mail has disappeared, I have been having problems for months, but now all is gone. I would like to uninstall and re install.Is this possible? OR what is the best e-mail program for IOS.I have to do something soon.

SO MAD I lost all of my e-mails, some I really needed.

Any help appreciated.

Sue