Mail 8.2 Stopped working With iCloud Account

Running an old iMac with Yosemite 10.10.5. My Apple Mail 8.2(2104) just stopped sending/receiving email from my iCloud account. When I try signing in to the icloud account through the mail app, it does not recognize my password. After multiple unsuccessful attempts at signing in to my icloud account, I tried removing and reinstalling the account but it continues to feedback "password not recognized" so now I cannot even create a new icloud account on this mail app. Yes I know, it's an old OS and Mail App . Any clues as to fixing this problem?