My environment
win10 system, virtual machine version 15.0, installed centos version 7.6, using a laptop
The first step: Use when centos startup keyboard Fn + F2 key to enter the bios settings, and then set ROM drive as the first boot in the boot screen, use the arrow to select ROM ... option, use the + - buttons to select the number who start priority "provided that you do not delete the iso image when you first install"
Part II: CD-ROM into the selection screen, select the troubleshooting option, select the option to rescue a centos system after entering, continue to continue to choose options, then OK, finally using chroot / mnt / sysimage
The third step: here is very critical, and I've tried many methods to repair bios boot partition but had no effect, so the direct choose to re-partition, first use the command parted / dev / sda print (when my hard drive sda) Check your what kind of partition
Yellow font format the partition is
Step Four: The gpt partition, for example, use the command gdisk / dev / sda, input, display all functions, input d, Input 1 (general bios root is the first to be based on individual needs, because time will enter p? show yourself to see), type p to see if deleted successfully, enter W, Y input
The following are examples of specific parameters to FIG according to their actual conditions
Step five: Re-establish bios root partition, re-enter the command gdisk / dev / sda, enter n add a new partition, enter 1, then OK, OK, that this 
time is to change the input EF02 bios root partition marked with a label, OK, and then W. exit.
Step Six: Performing post-installation successfully execute / sbin / grub2-install / dev / sda grub2 reboot reboot the system Andrews
Done