Debugging bootloop: Get an information log without ADB? Can an emulator help?
You will know if the kernel has hung, the led light stays on and not go further. 

As for your question, you need to be more clearer and specific as we do not know and since you posted a similar question before. You have not stated, what device is it, what android version is it, what kernel is it, all those are left out and thusly playing a guessing game here.


Is it the ramdisk not created correctly? 
Is it the address used for booting incorrect? 
Is the ROM built for VMSPLIT 3G and the kernel is built with VMSPLIT 2G or vice versa? 
What chipset is it? ARMv7 kernel with ROM compiled for ARMv6... 
Is the kernel actually booting? if yes, how do you know?


Far too many things here to speculate.

Edit: 

This bit is related to kernel level debugging.

The only way you can know if the kernel is indeed booting in the first place is to use a USB to Serial TTY cable with JTAG pinouts on a small circuit board clipped/soldered to the back of the device in question, and having the serial driver compiled into the kernel and treating the console as a tty device, and reading from it via minicom or hyperterminal to see the boot up sequence. 

As for bootloops,

The cause of boot-loops are down to the ROM itself, the initialization sequence is crashing somewhere. Now be careful :), the kernel in itself, can boot-loop also due to malfunctioning driver, panic's and reboots itself. 

So the question is, which is it?, Is it the kernel that keeps rebooting itself or has it got passed that stage and started loading the ROM? If its at the stage of loading the ROM, then something in the initialization is crashing, when the ROM crashes, it sends a signal to shutdown SurfaceFlinger, AudioFlinger, adb daemon, MedaServices etc. 

What you can do is this - in the init.rc, where you have this:

service console /system/bin/sh
    console
    disabled
    user shell


Change disabled to enabled, the next time, Android will dump you into the console, i.e. no familiar graphical interface.

Also, look for the  lines containing servicemanager, if the adbd daemon is listed there, take it out as what happens is the directive clause critical and onrestart will cause you to not have any adb facility!