Cannot Write on Drive C Error

Any simple thought please about this concern. I left my computer overnight with some of the opened programs running. When I went back in the morning, it was already in the off mode. When I started it, it performed scandisk and booted normally. The booting process took very long time and so I got irritated about it. I restarted it again and I was shocked with this error message that appeared on the screen
“Cannot write to drive C.”
Can you please tell me what the problem on my computer is? I have also initiated system restore as a recommendation from a friend but the same thing had been repeatedly happening. Any help will be highly appreciated.
Thank you.
