Hi, you need a game to boot the VB, it doesn’t show anything when powered without a cart. But if you listen carefully, you can hear a bzzzz when turning it on, it is because of the oscilating mirrors, at least you can know if it is working.
Also, the batteries should be as juiced as possible. I once bought a VB, then transfered over batteries from my other VB, and the new one wouldn’t work. I tried it later with an AC adaptor and found out that it wasn’t dead!