apple watch activation

Greeting,

I own an Apple watch, this watch has been purchased (second-hand). I used it for several months. One of my friends wanted to buy it. So, I reset the contents of my watch and give it to the him for test.
my friend gave the watch back to me, he did not decide to buy it. Now, I have not been able to use it as it seems to be forced to activate from an email I have not seen before. When he consulted about this email, he said he did not know it? is it the previous owner email??
Please help me as I have not used the watch for several months now.
For information (I have a backup copy in my iPhone for this watch)
Thank