Easy, I've had this happen to me a few times, reboot the phone, as soon as the green spkr button lights up, hold down #, the phone will go into factory reset mode, you will have to key in the following sequence quickly: 123456789*0#
Once that is done, the phone will ask you if you want to clear network config, select yes. After that you should be set, the default password should be the word 'cisco'