GoogleMail Outlook password invalid..why??

I have my Outlook set up with my GoogleMail account. And I have never had any problems with it. I recently set it up to where the password has to be typed in upon start up, and exit (to send). Everything worked fine, and one day it stopped accepting my password. It says that it is invalid. I know this is the correct password, because I am still able to log into my Google.com account with it. I have tried everything I could to fix it, but it won’t work. How can I get past this??