How to diagnose problem network (no connection)

Sometimes problem network may be occured on and off (intermittent.)

Firstly, check devices power, e.g. network switch, modem/router, computer. Can try to reset (switch off and switch on.)

then check the network cable (plug out and plug in RJ-45 (cable end) at network switch side, computer side, faceplate side.) (or change network cable to troubleshoot.) Faulty cable would be loosen RJ-45 clip.

(if any) and then check the keystone (CAT.5e) (normally is inside faceplate.) (or change keystone to troubleshoot.) (technical skill needed to change this part.) Faulty keystone would be dirty.

(if any) check the network cable along the ceiling (use Network Tester.)

after that check the network switch. (or change the network switch to troubleshoot.)

and check the network card. (change the network card to troubleshoot.)
if on-board network card is using, then add one network card to test.