If your drive is not being detected by your computer, first make sure all connections to the drive are securely fastened and connected to the motherboard. After verifying that the drive is physically connected, check that it's getting power from your PC. Most computers have a hard drive light that shines when the drive receives power. The drive will also make a noticeable spinning sound as it boots up.
The first step in checking drive power is to boot up with the power connected to the drive, then try again with the power disconnected. If there's a noticeable difference, then the drive is spinning up; if there's no indication of power, replace the power cord leading to your drive with another, and repeat the test. If your drive still does not power up, connect it to a second computer and repeat the testing. If it works, the problem could be with your motherboard or power supply; if it does not, it's an issue that can only be resolved by Seagate's technical support.