With all due respect to Joban. If you have the p506 (ie Thrive), his instructions do NOT work.
I had the same problem and tried using a kdz according to his instructions and this completely bricked my phone. Upon getting a new Thrive, I decided to try to flash again and got slammed into fast boot mode again.
The right way to fix a Thrive is as follows:
[HOWTO] Revive a bricked P500 on fastboot mode (the SHORT way) - xda-developers
PLEASE NOTE in STEP 5 that you should get the recovery image created by Raenye (here)
xda-developers - View Single Post - [Source] LGP506 (AT&T LG Thrive GoPhone)
Use that recovery image and it will allow you to get into Clockworkmod from which you should be able to reboot. I am assuming here that you never had a chance to flash a new rom, if you did flash a new rom you should reflash stock rom from here:
[ROM] LG Thrive P506 Stock FOTA 2.2.2 v11h - xda-developers
That should bring you back to stock.
Original discussion here:
[Q][HELP][URGENT] Help restoring lg-p506 to stock - xda-developers
Good luck