I can put the password in the window that pops up, but I still can't get the messages to download.

Thunderbird is not receiving incoming emails. I get a window that says " Login to server pop.centurylink.net with username nonnox15@centurylink.net failed. See image below. This occurs frequently but usually will download after a while. But today I can't get the messages to come in. I Can go to my firefox browser and get my messages from the centurylink website, but I prefer to use Thunderbird. Any help would be appreciated. I can put the password in the window that pops up, but I still can't get the messages to download.