A fail safe is designed to protect your rc from loss of signal from the transmitter and low receiver battery voltage. Depending on the fail safe it will either return throttle to neutral or a throttle/brake position you program.

If you have steering this means the fail safe is engaged because the receiver battery voltage is low. Charge or replace the receiver batteries (if they're standard AA's) and try again. If you have no steering this means the fail safe is engaged because the receiver is not receiving a signal from the transmitter.