HELP PLEASE QUICKLY

OK so i have an iphone 3g . Was backing up and then started to do the update . It started extracting the software , and started preparing the phone for update .

Then a message came up saying could not restore and update , an error ucured or something .

Now my phone is stuck in recovery mode and keeps saying the same thing when i keep trying to restore and update...

dont know what to do , my phone is dead , ahhhh

i have restarted computer and changed usb slots .

ANYBODY KNOW WHAT TO DO ???

KEEPS BRINGING UP ERROR 21