No Picture or sound - Grey screen?

I have a Matsui TV M19DVDB19 that started to lose TV channels. Signal strength was tested and found to be excellent so the fault pointed to an AV board problem.

I changed the AV board for a working one now all I get is a grey screen. Cannot access menu at all.

Only button on remote that responds beside the 'Standby' button, is the 'Aspect' button which when pressed causes the screen to go black momentarily then back to grey.

I suspect I have lost the firmware or software somewhere along the line. Any one know where I can download this and how do you update via the usb connection using a flash drive ?