I had a similar problem.
A spammer hacked my gmail account the other day and sent spam emails to everyone on my contact list. Google detected the unusual activity and temporarily disabled my account. I got up in the morning without knowing what happened, and got confused when Mail kept rejecting the legit password. I still don't know how the spammer succeeded in getting in when I had a strong password with numbers, capital letters and everything. But reading Google's help forum, it can happen if you're very unlucky. So if you're using gmail or something similar, you might want to check if your account is all right.
I had to google around for a few hours and tell Google my phone number to prove I'm the legit account holder...