how to restore arduino?

Hi everyone sorry this is my first post but i built an arduinome and i think the firmware got corrupt and is causing problems and not allowing me to reflash how do i go about restoring my arduino to factory defaults like the day i received it in the mail. thank you

You can either buy an ISP programmer (uses the six pin isp header) to reprogram the chip yourself or buy a new chip from Lady Ada, Modern Device, wulfden etc for around $5 and shipping.