I used 12v adaptor instead of 6v, now it has stopped working.

"Hi,

You most probably have a damaged voltage regulator and / or other input supply voltage components on the monitor's control board.

You will have to open it up and inspect it for any obvious damage.

Post some close up pictures of any damaged areas on the control board (hopefully it will be obvious but perhaps it won't be) and maybe someone here will be able to help.

Here's how to do it.

Adding images to an existing question"

It’s possible it used battery power during the time it worked and due to not getting a charge through the 6v power supply, it’s now drained. Try getting the right power supply and try again.