Mail on my Macbook Pro won't open properly so I can't access any of my mailboxes. Instead, a window asking me if I want to add an account comes up and when I click cancel, Mail closes. Anyone have any suggestions how to get Mail working again?

This morning I clicked on Mail on my Macbook Pro and it wouldn't open properly so I can't access any of my mailboxes. Instead, a window asking me if I want to add an account comes up and when I click cancel, Mail closes. Anyone have any suggestions for how to get Mail up and running again? I have an early 2011 Macbook Pro, with 2 GHz Intel Core i7 Processor and 4 GB memory. Storage capacity is 499.25 GB. It's running OSX Yosemite Version 10.10.5

Thanks

Elcatton