Office 365 University on Mac suddenly asks for activation

I purchased Office 365 University in May 2013 and activated it shortly after. Have used it since then without problems until the other day when it suddenly started to ask me to activate it. When I type in my product key, it says it is not valid. When I try to sign in to my Microsoft account, it says no such password but doesn't give me an option to recover the password. All the discussions I found online are not addressing my problem or are only useful for PC users. I want to talk to a real customer support person. How do I do this?