This could be due to one of the following:
1. The mail server maybe down or unavailable
2. The information supplied is not correct or a password has changed
3. Your not connected to the Internet.
Check your settings with that supplied by the mail provider. Check if the mail provider's servers are not down.
Best of luck
Thanks but I solved it by going to Mail - preferences- accounts-advanced-authentication in the advanced settings by changing to password and entering my password in. i am not sure why it had authenticated POP selected but worked when I changed to password so all good now and have a very full inbox to sort through now