My Mac Mail is not able to retrieve mail from my Yahoo/Verizon account.?

I have been retrieving email smoothly for months, and a few days ago I noticed my email volume plummetted. When I logged into my verizon mail, I found my lost mail. It looks as though the verizon account is not letting my email go. When the Mac Mail attempts to get mail, the program tells me it is downloading X# of emails, then nothing. So, I think my Mac is seeing the mail in the Verizon acct, but cannot get at it any more. Does anyone have a solution to this?