How to change the channel my Sony Trinitron TV turns on to?

Hi, previously I could turn the TV on by first turning the power on the digital box (using one remote controller)...then I pressed '2' on my Sony RM-836 remote control, and the TV screen would show a symbol of a small rectangular box with curved corners with an arrow going into the rectangle on the left of it. This symbol was to the left of the number 2 on the TV screen. Then a minute or so later, the picture would show on the screen, and the symbol & number 2 would disappear.

There's a similar rectangular shape with a left arrow symbol on the remote control underneath the power button...I think my housemate must have pressed it somehow...because now, when you turn on the TV, it shows the symbol next to '1' instead of '2'. You can only hear sound now, no picture.

Please does anyone know how I can change it back so it comes onto the symbol with the '2' again as before? I think that will make the picture come back again, as it will be coming on on a different channel than present.

thanks :)