How to enter into ISP mode through 'reinvoke ISP' in LPC1759
When you enter ISP, you "give control" to bootloader code in ROM. You can interact with it through UART interface (possibly also USB or something else, depending on the chip).

You might be able to regain control from code in the MCU itself if you use a watchdog. I'm not sure that's documented/guaranteed to work.

You need to use something that speaks this ISP protocol (e.g. lpc21isp), and you'll be able to issue commands such at detect, write flash, reboot.