Every fault is somewhere in a chain: the person, the application, the account, the device, the network, or the service. Your job is not to know every answer — it is to cut the chain in half, then in half again, until only one link is left.
Start by defining the problem in the user's words, then translate it into a testable statement. 'The internet is down' becomes 'Sarah's MacBook cannot load any website over Wi-Fi, while the reception PC (Personal Computer) on the same network can.' That single sentence has already eliminated the Internet Service Provider (ISP), the router and the switch.
Then change one thing at a time and write down what you changed. If you swap the cable, reset the profile and reinstall the driver in one pass and it works, you have fixed nothing you can repeat. In a 5–50 person business you will see the same fault again in six weeks; the note you wrote is what makes the second fix take four minutes.
Finish every job with a verification step performed by the user, not by you. 'Can you send a test email and tell me it arrived?' closes the ticket honestly. Then document root cause, fix, and verification — three lines is enough.