Fixing a N64 game

Try cleaning your cartridge with q-tips and rubbing alcohol. If that does not work you may need to open up the game and clean out the inside, and replace the battery inside of the cartridge if there is one.

First, open the cartridge using the appropriate screwdriver tool (it's an odd one). Check the cartridge for burnt out circuit lanes or dark discoloration on the ROM chip legs or body. If you see such discoloration, it is likely that one oft he chips took a jolt by either wrongfully inserting the cartridge into an already running machine or by resetting (yes, this may happen), because some diode actually failed and let backcurrent through. To verify cartridge ROMS and mappers circuitry are alive, plug in the cartridge without the casing, start the machine and measure for current on the ROM chip legs, and the mapper circuits (all IC chip legs) to make sure everything seems to be in order.

If it was burnt out you can not fix it, then the cartridge is wasted. If it indeed seems to be working however, while you have the casing off, make sure to use electronics cleaner to rub clean all the connector leafs on the cart board, benefits of doing this with cartridge opened is that you can reach the full length of exposed metal on the lanes, also tip clean any open lane (copper trace) on the board (some mappers use open trace connections for some reason).

Then replug and try again.

Last resort, desolder the ROMs and plug them into a a reader to dump and verify content, but that should REALLY not be necessary as the ROMs apperantly worked recently without glitching.