OSI Model: physical layer

check if the cable is plugged in
is the device you want to connect powered on?
is the device on the other side powered on?
are there any devices in between (e.g. switches) that need to be powered on?
does any LED for the port light up?
- for network ports: figure out what the lights mean. Do they blink? How do they blink?
can you measure a signal with an oscilloscope when you poke the data transmission lines with it?

how to debug