Grey screen or flashing power happened all too often when I was a kid. When it happened, I just took the game out and blew in the cartridge. Most often it was all it took to get it to work.
From what I know now, it was probably a bad idea to blow at the connector in the cartridge with your mouth; warm moist air, perfect for corroding metal. I think it would be better to clean the connector with isopropyl alcohol. If you have changed the cartridge connector in the console, I recommend you to clean the connectors on the game.
