"Attached debugger" is vague.
It may be that it is a powering issue and it is powering the target somehow. You should diagnose that quite easily.
It may be that your nreset is floating and a debugger pulls it up. Another easy to diagnose, embedded 0b101.
Then there may be some software problems. LPC1769 comes from power up with slow RC clocking but a debugger can spin it up to something faster, like external quartz + PLL. No debugger - no spinning up. Make sure you run PLL config at startup.
And the final idea - binary made for debugging would most likely not work without "debugger attached". So you cannot disconnect a debugger, reset target and enjoy running a binary on Cortex chips. That is because most likely binary made for debugging is linked against semihosted libc. Such libc uses asm("bkpt") which makes cortex core hardfault.