Apple ID password is correct but not accepted

Somehow, my macbook (Macbook Air 13 inch, Early 2015; running Yosemite v10.10.5) has become uncoupled from my Apple ID and I can no longer download from the App Store. My password and email are correct: I used it to log into this forum. However, when I use it to log in to the App Store it says the password or email is incorrect. Has this happened to anyone else and is there a known solution? Thanks in Advance.