Hello _____,

My name is Billy and I've taken over your case regarding not able to boot. Please read over the below questions/instructions, and try to answer/follow them the best you can.

--Can the system boot into an older kernel from the grub boot menu?
       https://access.redhat.com/site/solutions/226423

--Has there been any changes to the hardware, or firmware, also has there been any updates applied since the last successful reboot? If so, what were they?

--Are there any third party kernel modules installed previously on the server?

--If the system can reach the GRUB menu, below are the steps to get more verbose boot messages:

    1) Interrupt the countdown at GRUB.

    2) Highlight the kernel you are attempting to boot into by using the arrow keys, and press "e" to modify the kernel parameters of the selected kernel.

    3) Remove all instances of "rhgb" and "quiet", you may have to arrow to the left to find them if they are not at the end of the kernel line. Then append the following "debug".
            - For RHEL 7, you will need to arrow down to the line that starts with "linux16" once the cursor is on the linux16 line press the "End" key to move the cursor to the end of the line.
            - For RHEL 6 and below, you will be displayed another window, arrow down to the line starting with kernel, and press "e" again this will take you into the window to modify the kernel parameters.

    4) Boot with the changes added above.
            - For RHEL 7 simply press ctrl+x
            - RHEL 6 and below press "enter" then press "b" to boot with the changes applied.

    5) If the Plymouth Splash screen appears (the red and white loading bar), remove it by pressing the "ESC" key or an arrow key.

    6) Once the server has failed to boot there will be more log output on the screen than before. Please gather screenshots/pictures while it sits at this screen, then press shift+pageup to scroll back through the log output, and take more screenshots/pictures each time you page up. The more screenshots/pictures that you can take, the higher the oppurtunity that the logs will show us what, and when the actual error occurs during the boot process.

--Along with the more verbose boot output, we will also need you to boot into an installation media and enter rescue mode, from rescue mode you can grab a sosreport for analysis.

       How to generate sosreport from the rescue environment
       https://access.redhat.com/articles/3405661

       How can I provide large files (such as a vmcore, big log files, or a large sysreport) to Red Hat Support?
       https://access.redhat.com/kb/docs/DOC-2113

Please feel free to update the case with any comments, concerns, or feedback on the above.

Best Regards,

Billy
Associate Technical Support Engineer, RHCE
Red Hat, Inc.