NEED IMMEDIATE HELP PLEASE!!

I would suggest you first check your bios and verify the hard drive ordering ie. what you consider hard drive 1, 2, 3 ect, then double check to verify the hard drive with your OS is the first in your boot order.

If you happen not to see both your hard drives in the bios, something is still unplugged in your case.

Sounds like some cables were not completely reconnected. I would go back and inspect each one.

I can’t begin to tell you how many times this has happened to me and I am a expert at fixing computers. The most common causes of this problem are the cables are not connected properly or even the power supply cable not connected at all. The data cables not connected to the same drives as before thus altering the boot order. The data cable being pulled out of the motherboard. It is also possible to damage or brake cables but this is unlikely.
Problems like this are normal when you start playing about with the insides of your computer and how quickly you can sort out these problems is a test of your computer skills.
Also check that you have no USB memory devices connected to the computer when it boots.