It sounds like you haven't attempted any troubleshooting. Try restarting in Safe Mode, this will clear some caches it's possible one has become corrupt. If you have done some troubleshooting please advise what you have done so far.
Yes I did attempted troubleshooting by Safe Mode.
System Preference ---> User & Groups
Automatic login dropdown menu shown as "OFF" so i changed to My "user name".
At first, I tried to changed Automatic login, Preference window froze on me. I tried so many time
Question is how come recently Mac asked me to sign in, i haven't set this option at the beginning.
Anyway, this issue has been fixed. Thank you for responding my question.
Still one issue remaining, OS take long time at start up and longer time at shutting down.
Long startup times and long shut down times can be cause by many things including a HD failure. I would recommend running Apple Hardware Test in Extened Mode to see if your HD is reporting any issues. This test will take some time so be patient, if it doesn't report any errors there are some other troubeshooting steps that can be done.