"Authentication is required" error

After working just fine in the past, this morning my emails refuse to send with an error message: “Authentication is required before sending (R0107005)”. and now the program refuses to even connect with my server. Help?!!! I am unable to reply to mails as well as to send new ones! 
Ok, I have now rebooted, restored and the diagnostics on the account say it is fixed. So I wrote another email and it went back to telling me the same error…and refuses to send. HELP!
So, all day…unable to send emails. Would appreciate some help.