As others have said, the OP was too vague to really determine what the problem could be. But here are some general ideas:
- Try plugging into a different USB port or a different computer. USB ports (or whole USB buses) don't go bad often, but it does happen.
- It could be a power issue. If your drive takes a separate power cable, try replacing it with another adapter WITH THE EXACT SAME DC OUTPUT VOLTAGE (12V, 5.5V, etc.) and an amp/milliamp rating >= your current adapter. Those
universal power adapters can be useful for this. If your drive is bus-powered (no separate power cable), try moving it to a different port on your computer, or try it on another computer.
- If you can tell that your drive at least powers up, put your ear against it while turning it on (make sure it's connected to the computer as well). A normal drive will spin up, initialize (a single, kind of electronic-y click as the heads unpark) once it's up to speed, and then start accessing data (lots of smaller, shorter, irregular clicks). If it doesn't spin up or if it only spins up and you never hear any clicks, your drive may be underpowered (see the power stuff above). If you hear any repeated, regular clicks from the drive , you're pretty fucked*.
- It's possible that the drive enclosure's electronics have failed. You can try (carefully) opening the drive's external enclosure and removing the actual hard drive from the inside - depending on the drive you may just need some small screwdrivers, or possibly some pry tools. Then, you can place the drive in a
hard drive dock and connect the dock to your computer. With any luck your computer will see the drive. If it doesn't, you may try it in another computer (even with a different OS). If it still doesn't work, you're pretty fucked*.
* By "pretty fucked" I mean you're either totally SOL or will have to take your drive to a data recovery place. As others have posted, it is not cheap (think at least $500 or so) because they disassemble your drive in a clean room environment, remove the platters, and analyze the drive to see what's good and what isn't. There's not really a way around them seeing what you have, kind of like taking your film of nudie pics to get developed back in the day. So you just have to decide if it's worth the cost and potential embarrassment.
Good luck. We're all counting on you.