Scenario : My internet at PC is not working (Suppose a single pc with leased line) What should be troubleshooting steps from PC to leased line?

check the cable, line connection,

check pc internet configuration 

Check first if the correct cable is being used. Then verify if your ISP is giving you their service or not.

I would first check the cable that connects your PC for any physical damage

Run a diagnosis

Reboot the Access Point

Check for software malfunction

Check if the problem occurring to other devices as well

Call the provider for check up/maintenance

those are from the top of my head .. in a real-life problem  it might not take all these steps, or might take more. no one knows