Hi Ram,
Thank you for posted i am here to help you today. Possible reason you are getting this problem is due to shutdowns as a security measure to prevent unauthorized access, repeated failed attempts, incorrect password entries, Windows update issues.
I'll be asking few questions to know more about the issue.
May i know the device you are using?.
May i know what credentials you are using like Pin, password or etc?.
Are you using a personal, organization or school device and account?.
If you are using a organization / school account and device reach out with your IT Administrator / IT Helpdesk they will assist you regarding your problem.
If using a personal account and device
keep the power of your laptop connected. You don't need to do that for the above 2 hours. The screen is dark or black because your system's power protection plan is to save power, not power off. You only need to wait for 2 hours then try to log in again.
I hope the above will be helpful to you. Please feel free to let me know how it goes, and I will do my best to help you.
Best Regards,
Jansher