System freezes while unlocking the screen after one hour or so

Hello Friends,

My home desktop computer (Lenovo A340 24IWL AIO) freezes at unlock screen if the desktop was kept unattended for an hour or more. As soon as we enter the password and hit enter, KDE plasma loading screen with progress bar appears, and then freezes. I am seeing this after a system upgrade yesterday. Any pointers to trace the reason would be very helpful. Thank you in advance.