EMERGENCY -- Turned on iCloud and now Mac Mail is "Read Only". All my messages are gone and can't figure out how to get back to my normal email... Help!!

I turned on my iCloud and all of a sudden I have no more emails in my entire Mac Mail (usually has over 100K since it is my work email). My account is also now completely disassociated with the email. I need this resolved within the hour if possible. I can't restart or redownload my Mac Mail because I need all my past emails to remain in my inbox.

HELP!!