my email address already verified for another Apple ID?

So, my email address stopped working yesterday sometime and I discovered that somehow it was locked. No idea how this happened.... I managed to unlock it, reset the password etc but now I cannot register it with my Apple ID (which I'm 99% sure it was registered before)

I've tried on my ipad, phone and laptop

The error message is: Email address is already verified for another Apple ID

Should I be worried? Has someone 'stolen' my email address??

And importantly, how can I associate it to my Apple ID again?

Any help is greatly appreciated............