[Atrix 4G Q&A, Help & Troubleshooting] android-DEP:
which rom did you flash last on your phone? Was it gingerbread or CM7 based or CM9 based?

Depending on your answer to that question, you can try a few things to restore your phone using fastboot.

If it was let's say some CM7 based rom, you can locate a stock CM7 kernel and download it. Extract the zip file to pull out the "boot.img" file and put it inside the same folder you're running fastboot.exe from.

Once that's done, reboot your phone into fastboot mode, and connect it to your PC via usb cable and real quick just type "fastboot devices", that command just shows you which devices it's currently connected to via the fastboot interface and you should see your phone listed with it's serial number showing which tells you that you can continue the procedure..

then do a wipe of any data which might interfere with the new kernel by typing "fastboot -w", don't worry, you won't lose any personal files like pictures or contacts..

after thats done then type out "fastboot flash boot boot.img" which will flash the new kernel onto your phone.. phone's not booting past the initial Dual-Core screen can sometimes be attributed to a bad flash of a rom, kernel not properly being written to the device in cases where the updater script couldn't mount a partition to extract the boot.img to before writing the raw data to your device. The updater-script which is just like a batch file, it just goes down line by line running commands and does not halt on errors to let the user know that something failed. So if something failed during the flash process and you didn't successfully flash some parts of the rom normally, it could still finish the installation and all you'll get it Installation Completed type message where you think it's all good but in reality some parts of the installation didn't complete.

Perhaps the rom didn't properly perform the necessary prep work of wiping cache and dalvik cache. Not exactly always an issue as it depends on the circumstances of what you're installing and what you had installed prior.

I've had a few roms here and there that put me in your shoes, the solutions I've always found to work was to first try something basic like wiping the cache of the device and reboot. If that didn't work, then this is where you could try flashing a new kernel.

Since you did the proper wipe of data, you can make sure the kernel gets installed properly as you isolate the installation from the entire process.

It takes 3-5 seconds anyway for fastboot to flash files anyway so theres no harm done anyway so long as you get the proper compatible files for your setup. Reboot the phone afterwards and give that a shot.

Unless you tried to flash a PDroid patch on your phone, which requires you to have a working rom installation to begin with from which you pull files to patch so you can flash them over the current installation, you should be able to boot.

If you still cannot boot, then reboot into recovery and open up the ADB interface in command prompt and run this command

adb logcat > "c:\logcat_dump.txt"

and just kick back for a few minutes while it dumps the entire log into the root of your drive in a text file named logcat_dump.txt... press the Windows symbal key(its between the CTRL & ALT buttons on the left side of the keyboard), while holding that press the letter R to bring up a run window box where you type in "%SystemDrive%\logcat_dump.txt" and hit enter... post the log here and you can get more feedback on where the issue is