Your hard drive will likely let you know. Hard drives fail for numerous reasons and exhibit different symptoms as they do it. For example, the most common hard drive failures include mechanical failure, electrical failure, firmware corruption or damage, and logical corruption. The first three types almost always indicate a fried hard drive that will need to be replaced. However, you may be able to recover data from them by sending the drive to a professional data recovery center.
Mechanical failures are pretty easy to identify thanks to their telltale "clicking" sounds. This means that a mechanical part is broken. Fortunately, you can send your hard disk to a data recovery "clean room" company and have much of your data extracted. This option is quite pricey.
Hard drives fail due to electrical failures as well. When you turn on your computer, the hard drive will not spin at all. The computer won't recognize the disk at all. If your operating system is on the failed hard drive, it won't load either. This type of failed drive often yields recoverable data if sent to a recovery facility. Again, an expensive fix.
Firmware corruption and damage is another type of hard drive ailment that requires professional help. Firmware is an embedded chip within the hard drive case containing codes for controlling the disk. If the instructions within the firmware become damaged or corrupt, the drive will fail. Symptoms include a spinning disk when the PC is turned on followed by a non-recognized disk or a system hang during boot up.
Logical hard drive failures are among the easiest hard disk errors to recover from. In this case, the hard drive is physically fine. However, the information contained on it may be causing the problem. For example, fragmented disks, file corruption, and misinformation in file allocation tables are common logical drive issues. Symptoms include disk errors, slow performance, computer errors, can't find file (or the operating system) errors.
If your hard drive is truly fried, you'll need to replace it. If this same drive was your computer's primary drive, you will also need to reinstall your operating system and programs as well as restore any data from a backup. If you don't have a backup, consider sending your drive to a professional for data recovery.
For logical hard drive errors, you still have hope. First, data recovery is possible through the use of tools such as Data Recovery from ParetoLogic Software. Second, you may be able to repair the cause of logical hard drive errors using a good registry cleaner or built-in disk repair tools like disk defragmenter.