Four things sit between your files and the screen — cable, port, power and the bridge board inside the case — and three of them are cheaper to rule out than the disk. Work outward before assuming the worst.
A drive spinning quietly and never appearing points at the electronics. One that clicks, ticks, or spins up and stops points at the mechanism. Those two situations call for opposite responses.
A drive that does not appear could be failing, or could be perfectly healthy behind a fault in something else entirely. Working outward from the drive eliminates the free possibilities before anyone pays for the expensive one.
Start with the connection, then the port, then the machine, then the drive itself. Most enquiries that reach us on this turn out to be resolvable at one of the first three, and we would rather tell you that on the phone than take a fee.
The one thing worth avoiding throughout: if Windows offers to initialise or format the disk at any point, decline.
A different cable. USB and SATA cables fail silently and are the single most common cause of a drive that suddenly is not detected. Try two.
A different port, and a rear one. Front-panel USB frequently cannot supply enough current for a 2.5-inch portable. Rear ports on a desktop connect directly to the board.
The power supply, on desktop externals. 3.5-inch units use a separate mains adapter, and those fail more often than the drives they power.
Another computer entirely. Eliminates drivers, controller modes and operating system faults in one step.
Then the hardware is working and the file system or partition table is not. Do not initialise it — that destroys the structure describing your files.
Hardware or firmware. On a mechanical drive listen for whether it spins; silence usually points at the board, which is the cheaper end.
Intermittent detection is frequently the stage before permanent failure. Copy everything off now while it is still cooperating.
From £300 +VAT for a drive, fixed in writing after the free 48-hour diagnostic.