After years of use, my MS-Office 2013 suddenly claimed to be not yet activated and threatened to stop working. I tried to activate it, but this proved impossible. (First it was 'this code is already registered, then 'not a valide product code'.)
However, my fiddling must have helped somewhat as next day everything seemed alright again: no error messages when starting any Office application and when clicking 'Account' I see the reassuring text 'Product Activated'.
Yet I fear not everything is right. When clicking 'Additional licence data' I DO get a warning against an alarmingly pink background: 'Activation required'.
So: is my Office safe or not?
Mabel

Thanks for the elaborate answer. It doesn't really help me.
1. "If Office activation fails, you'll see Unlicensed Product or Non-commercial use / Unlicensed Product in the title bar of your Office apps". Did you really read my posting? I see no such thing in my title bar!
2. I won't "sign-in", never have, never will. Didn't buy Office 'using an email-address', just used my bank account.
3. I may indeed have 'multiple copies' of MS-Office 2013 installed. I can see two entries exactly the same: 'Microsoft Office professional Plus 2013'. Please tell me: how can I tell which one 'I don't use'?
4. Your "Step 3: Check your Office 365 subscription status" is beside the point; I have no subscription to MS-Office 365.
Mabel

I have a similar problem. Following recent purchase, Office 2011 for Mac (Word, Excel, Powerpoint in particular) repeatedly but not after every load or usage, puts up dialogue box asking for the product key. I can enter the 25-character key now or 'later' (the latter tells me I've activated the product even though the sequence will be repeated), but, as ever, "the activation server is temporarily unavailable".

The product still has at least one of its original five licenses, the product key is correct, the product runs as expected, but the activation nuisance persists. Anybody found a work-around for it ?

Did you buy one time Microsoft Office license or is it a cracked version?

It was a second-hand online purchase with at least one (probably four) unused licenses.

Elsewhere you will find a thread from me on contacting Microsoft UK (--> US) without using voice calls. Deafness makes voice calls very difficult for me, an online chat window being the best option.

A "solution" which worked for Office for Mac 2011 was to register for an Office 365 account (see separate thread referring to Office 2010 for Windows), which enabled me to run and save in Office (Word, Excel, PowerPoint) applications which had been enforced read-only.