Problems logging into iMessage on iMac

Our iMac recently went through an update and when I went to sign onto iMessage via my iMac, I was prompted to enter my password to my Apple ID. Following the approval of the password entered, I was prompted to enter the password to my email address, which I then entered in as well; however, after that I was given an error message that read "Could not sign in to iMessage - The user name or password was incorrect. Try again." I have tried numerous passwords, including the password that I KNOW belongs to my email address and still no success at logging on. Any suggestions as to have to correct this issue so I can begin to use iMessage on my iMac again? Thank you!