The OSI model gives you a logical path through the network stack. Start at the bottom and work up. If a lower layer is broken, higher layers cannot function.
Bottom-up approach:
Physical. Is the cable connected? Link lights on?
Data Link. MAC addresses correct? VLANs configured?
Network. IP address assigned? Default gateway reachable?
Transport. Port open? Firewall allowing traffic?
Application. Service running? Logs showing errors?
When stuck, always verify physical connectivity first. You can also work top-down if you have strong clues about the failure layer.