First, the lights. A Synology with a static or blinking orange status light, a QNAP with a red status light, a WD with a red light, a ReadyNAS with an error on its display, a Buffalo showing an E-code: these are units with a fault, and the network question is answered. Power down and go to the page for the light. A unit whose lights are normal has the network to answer for.
Then the cable and the port. Swap the cable for one you trust, and the switch port for another. Look at the link light on the NAS's network socket; no light means no link, and no link means the switch, the cable or the NAS's port. Look at the router's list of connected devices for the NAS's name or its MAC address; if it is there with an address, the unit is on the network and the problem is in Windows or macOS, which is a mapped-drive or a name-resolution question rather than a NAS one. Try the address directly in a browser.
Then, once, a power cycle, done properly: shut the unit down from its button or its dashboard if it will answer, wait, and start it. Once. A unit that comes back is a unit that had a stuck service. A unit that does not come back, or comes back with a light that has changed colour, is a unit with a fault, and the next power cycle is one too many. From there it is a parcel, not a network problem, and the drives in it should not be touched.