Have you ever been able to debug your system? If so what changed, or what code did you program into the MCU, before it stopped working?
Is this an off-the-shelf board, or your own board?
If your own board, please check out - https://community.nxp.com/message/630601
What debug probe are you using?
What version of LPCXpresso IDE are you using? And on what host OS ?
Please also double check that you following the process for booting the chip into ISP mode correctly - you must be pulling the ISP pin as the board is reset/powered up.