'Not recognised' can mean two different things, and they have different fixes: the PC doesn't see the device at all, or it sees it but can't use it. The quickest way to tell is to look in Device Manager — if the device appears there (even with a warning triangle), the PC sees it and the problem is software. If it's completely absent, the problem is physical: the port, the cable, or the device.
Try another port and another cable
Move the device to a different USB port. On a desktop, use a port on the back of the case, directly on the motherboard — front-panel ports and USB hubs are the most common source of 'works sometimes' faults, often because they don't supply enough power. If the device has a separate cable, swap that too; a frayed data cable is a classic.
Give it power
External hard drives, in particular, need more power than a single front port often provides. If the drive has its own power adapter, use it. If it's bus-powered (one USB cable only), try a powered USB hub. A drive that spins up then stops is a textbook power problem.
Check Device Manager
Right-click the Start button → Device Manager. Look under Universal Serial Bus controllers and under Disk drives. If your device shows with a yellow warning, right-click it → Update driver, and if that fails, Uninstall device, then unplug and replug to force a fresh install. If it's not listed at all, the fault is the port, cable, or the device itself.
Give it a drive letter
Sometimes the drive is connected but has no drive letter, so File Explorer won't show it. Open Disk Management (right-click Start → Disk Management). If you see the drive but it has no letter, right-click it → Change drive letter and path → Add, and pick a letter. It will appear in File Explorer immediately.
Is the device the problem?
If the device works in another PC but not this one, the port or Windows side is at fault. If it fails in every PC, the device itself is dying — with USB sticks this is often the end, and with external drives it may be a failed unit. If the data on it matters and it's not being read, stop trying (each attempt can make data recovery harder) and take it to a data-recovery service.
Front-panel ports and unpowered hubs are responsible for a shocking number of 'my USB doesn't work' reports. The back of the case is always the place to test first.