TV goes off & on?

I have a sony tv and cable. When the cable was installed the tech could not get the remote to work so he told me to use the on/off button on the tv. Well, now when I push the buttons it comes on for a second and goes off and once it made a loud sound. I can use my remote but it does not go all the way off. The cable company said it is the TV but the picture is good when I use the remote. What do you think it is>
Patty