Sounds like what would happen if you had an add-on video card installed, and then the disassembly/reassembly caused it to default back to the onboard video. Of course, that's just a shot in the dark unless we know the specs of your machine and exactly what you did in the process of taking it apart and rebuilding it.
ITS HARD I DO NOT HAVE A CLUE HOW TO PUT IN THE RIGHT PLACE
UNLESS I HAVE PICTURE
Seeing as you don't have a picture, I'm assuming that this is a prefabricated system that did not come with a motherboard manual. That being said, there are two ways to connect your drives: IDE or SATA.
SATA is the narrow cable with and "L" shape pattern on either side. If your HDD is connected via SATA, start with reinstalling the cable into SATA 1, followed by SATA 2, and so forth, until you've tried all SATA slots. If the cable doesn't install, flip it and try again.
IDE is a wide, ribbon-like cable that has an elevated section in the middle of the connecting ends. If your HDD connectes using IDE, the easiest way to figure out which of the IDE slots it plugs into is just as simple. Carefully remove all IDE connections to the motherboard. Notice the length of the connecting end, match the elevated section with the void in the IDE slot on the motherboard. Trust me, this is easier than it sounds. Like SATA, IDE can only be installed one way, just be mindful of the pins, as they are easy to bend.
Speaking of pins, if your HDD is an IDE interface, inspect the pins on both the HDD and the mobo.
On the right hand side (in red box) you will see to 2 connections 1 blue and 1 black these are your IDE connections (your MoBo does not have SATA connections) Just swap your cables and see if it boots.
If it still does not boot you need to swap the cables at the other end (CD and DVD players) or if you have more than 1 harddrive swap the cable one to the other.
Hope I explained that good enough for you. Let me know how it turns out.