Hard disk not detected

take the sata cable out of your DVD drive and insert it into your hardisk. if it works it could be something wrong with the other wire. next attach the other wire to your dvd drive, if this fails it must be because your power supply is not supplying enogh electricity.

Check if your HDD is set to master, slave, or cable select.

I would do as Mi1ez suggests and verify that the m/s jumper is set correctly for your drive in a single drive system. Unfortunately the 'single drive' setting varies with your hdd model so you'll need to check the manufacturers support site for that.

Also go into your bios and check that its set to boot from ide/atape hdd first.

yes, i would consider you to check whether your hdd is set to master, slave or cable select. had it always been this way?