Boot your Nokia G300 back into Fastboot mode using your PC terminal.
Run fastboot reboot fastboot to enter userspace fastboot, then wipe the device. nokia g300 custom rom
Disclaimer: Modifying your device's software is risky. You are responsible for any damage, loss of data, or invalidation of warranty that may occur. If you're interested, I can help you find: Boot your Nokia G300 back into Fastboot mode