Before you conclude that the disk itself has failed (the worst possible outcome), you should work outward through four areas that can cause problems: the cable, port, power supply and the bridge board inside the case. Three of those are cheaper to eliminate than the disk.
If your drive spins quietly and will not show itself, the problem probably lies in the electronic area of the unit. If, however, the drive makes noise (for example, clicking, ticking, or spinning up and stopping), the problem probably lies within the mechanical components of the unit. The above two scenarios require completely different approaches to resolve.
An unrecognised hard drive may have failed, or it may still be working correctly but behind a failure somewhere else. By eliminating each potential source of trouble, working outward from the drive, you are ensuring that you rule out the free possibilities before you pay for the expensive one.
The best way to ensure this occurs is to start with the cable/connector, then move outward to the port, followed by the machine (or computer) and finally look inward at the hard drive. We find that in most cases where users contact us about this type of situation, their problem can be resolved at one of the first three items listed above, and we'd rather inform you of that via telephone than charge a fee.
Avoid doing anything with Windows that may lead you to “initialise” or “format” the disk at all costs.
A different cable. Your SATA or USB cable can fail without warning. It’s the number one reason why your hard drive suddenly won’t be recognised. Try using two different ones.
A different port, and a rear one. Front-panel USB frequently can't give your 2.5-inch portable enough juice. If you’re trying to hook up your portable via front-panel USB, try a rear port instead, as these connect straight into the motherboard.
The power supply, on desktop externals. Your 3.5-inch unit uses a separate mains adapter (which will break down more often than the hard drive itself).
Another computer entirely. This will eliminate any driver issues, controller mode problems and OS-related errors in one step.
Then the hardware is functioning properly but the file system or partition table isn't, so don't initialise it. Initialising a disk wipes out its internal structure which describes where each of your files is stored.
Either hardware or firmware. Mechanical units should spin if they're working, so listen for it. Silence usually means there's something wrong with the drive's board, which is the cheaper end.
Detection failure is frequently intermittent before it becomes full-on. Get everything copied off while it still works intermittently!
The cost (from £300 +VAT for a drive) is fixed in writing after the initial free 48-hour diagnostic.