My Mail has suddenly stopped working

My Mail app has suddenly stopped working - or rather one account on Mail has overnight stopped working for no obvious reason. I get a general message headed Online Status and which reads: "There may be a problem with the mail server or network. Verify the settings for the account or try again. The server returned the error. An SSL error has occurred and a secure connection to the server cannot be made." How do I correct this please? I can't see any obvious error in the account settings. I have tried to take the affected online via Mail/Accounts but it won't let me. I should add that I can access my email via webmail and that I can access the internet for every other form of browsing. And my other accounts on Mail work fine.

Help please!