"Check Tyres - Then Reactivate" ????
 

Yesterday I had low pressure in my right real tire on my 2005 SLK200. This was indicated by a red warning that said something about not enough air.
Anyway, I topped up the tire and went on my way. The next time I started the car a message popped up saying "Check Tyer - Then Reactivate".
I don't know what this "Reactivate" means and how do do it. The tire pressure is fine so how do I "Reactivate"?
Thanks in advance.