Tektronics display

Hi, I have a Tektronics display 21HV8SA, similar to Viewsonic G810 / Panasonix TX-D2162. There is four buttons at the front (writen: 1 < > 2) + on/off button. With the four buttons, you can tune the brightness, contrast, colors, select the input ... What has append, I've no idea, but when I push a button, I get a lock on the screen and I'm stuck there, no way to change my inputs! Any idea? I've tried to push all the buttons but I always get that lock on the screen.