I have two N64 systems and one won't work

The N64 doesn’t have an O/S so it’s up to the games to make it do anything other than a black screen.

Try rubbing a bunch of isopropyl on a game’s pins and then jam the game in and out of the slot several time (this cleans the contacts inside the cartridge slot). After I did that, the games would sometimes work, I had to push em in and out a few times to get them working.. So I took the console apart, pulled out the cartridge slot (it pulls straight out of the motherboard), cleaned all around the cartridge slot area with isopropyl and a cotton swab (cleared a bunch of yellow crap and dust and grime build-up). Works every time now.