I have a Dell PowerEdge R420 which has installed Centos 7 and was working fine. One day I removed the disks that Centos 7 was installed (2 disks with RAID-1 layout) and I installed another clean disk to the rack in order to install Red Hat Enterprise Linux 7. For reasons I cannot describe here I wanted to have a disk with RHEL 7 installed so I can use it with another server. The installation of RHEL was also fine, but when I tried to put back the initial disks with Centos 7 into my initial Dell server I saw that I was stuck in Grub rescue mode. I used these steps in order to reinstall grub and know I am stuck into emergency mode. I see the following message:
Welcome to emergency mode! After logging in type "journalctl -xb" to view system logs, "systemctl reboot" to reboot, "systemctl default" to try again to boot into default mode. Give root password for maintenance (or type Control-D to continue).