Are you absolutely sure there’s a game cartridge inserted? The VB doesn’t do anything without an inserted cartridge, unlike the original Game Boy for example.
Also, it takes a couple of seconds for the image to appear after you turn the system on.
see if there are any dust covers in the cartridge slot sometimes people will stick them in there and they are hard to see, also clean the contacts on the battery pack and controller then test the controller with a volt meter to see if its that or the unit itself.