1. Enter the password to log in to your account;
2. Check the error log at startup through journalctl -p err -b;
3. Edit the fstab folder vi /etc/fstab;
4. Find the abnormal line in the error log, and add at the front #;
5. Reboot;
Open the CentOS7 page and the following appears:
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
（？？ Control-D？？？）：
Solution:
1. Execute runlevel and display unknown
2. Modify the default startup level
a. Delete first: mv /etc/systemd/system/default.target /etc/systemd/system/default.target.back #Rename the file
b. Create a soft link file: ln -sf /lib/systemd/system/multi-user.target /etc/systemd/system/default.target
or
ln -sf /lib/systemd/system/runlevel3.target /etc/systemd/system/default.target
You can also use the systemctl command: