My computer cannot open! Once I restart it, it stop this screen forever!

Can be lots of reasons. I'd start with:

Try a safe boot.

Shutdown your machine. Hold down the shift key. Poweron. Wait awhile; wait awhile while you harddrive

is being checked. Also, what features and apps does safe boot leave out?

http://support.apple.com/kb/ht1455

At this point in the startup process, your Mac is running the loginwindow application, which is one of Core Services of macOS.

What this app does, during this time is:

• Check your password against information in Directory Services
• Daemons in the /etc/mach_init_per_user.d are loaded.
• Your customized environment is set up.
• Files are set to show the appropriate permissions and privileges for your account and status.
• Unix-style environment variables are checked and executed.

Somewhere along this process something went wrong. Most likely, is could be your particular user account is corrupted.

You have a couple of choices to proceed:

1. As rccharles has recommended, try starting up your Mac in Safe Mode.
2. Boot up normally, but when you get to the logon screen, try using another account. For example, use your Administrator account.

I'd follow the other two responses.

My question, is how long was it "STUCK" before you got worried and did anything, then what did you do?

I've seen FYI, some normal install take a VERY very long time, and then others not. I've seen people who have quit/stop before they should have and just wanted feedback from you as to the general timeframe.

Mine I waited 40 minutes before forced restart and then an hour then 3hours.