Got a custom recovery? Flash a different rom.
Or flash a stock Samsung rom via Odin.
Odin will restore all partitons (system, data, recovery, bootloader, etc) to stock, while the zip file will sometimes flash to certain partitions, so it won't be fully stock. The stock file you have may even require a custom recovery to work, which may explain why it doesn't work with fastboot commands.
Which zip file are you using and what method are you trying to do to flash it?