Gateway model 834gm won't reboot.The gateway symbol comes up,and then won't intialize or go to windows xp

You say reboot. How does it perform from a cold boot? If it doesn't boot in either case and the computer was previously working properly and suddenly stopped, the most likely culprits are a failed hard drive or changed BIOS settings. However, if POST (http://en.wikipedia.org/wiki/Power-on_self-test) is not completing then memory or another component may be the issue.