Hello Peter, you are right it was related to the PINCODE. This morning when I switched on the TV again I was asked to set the PINCODE and when I set it, it started working normally.
Then I searched for the reason and I found out that yesterday after the power failure, my son had tried to set the PINCODE unsuccessfully by 5 attempts and the TV was locked, not responding to any command for 3 hours.
(This is just for an experience to the forum)
Thanks and regards. Lakis