Computer starts up, Moniter does not correspond

[RedConverse]

I've recently had trouble with my CDROM drive, so I opened tower (something I haven't done before and have no experience with), and tried to see if there was something internally wrong with it.
After opening it and seeing that I don't have a clue how to dismantle everything, I decided to just vacuum all the built up dust inside, and then I sealed it back up, hooked all the wires back up. (also, when it was open, I did accidentally knock some big circuit board thing along the side ajar, although I think I've got that back in place).
I turned the tower on, it initially made a loud, unusual noise, like maybe something was loose. Also, nothing came up on the monitor (except for its standby screen). I switched it off and on. Nothing changed, so I gave a light but firm smack upon the top of the tower. The noise stopped, but it still doesn't work. Everything sounds normal in the tower on startup, except for one loud, sustained beep that the tower emits when I start it.         
Any help is greatly appreciated!

[Aegis]

What operating system are you using on the computer?

Do you get any output on the display at all when you start it up?

[Kurtiskain]

When you say knocked ajar..you mean bent vertically or like it had come out of a socket?

[RedConverse]

I'm using Windows XP. The only thing that shows up on the monitor is an icon says the monitor is working, and to check the cables and computer.

 It's a circuit board thing that's flat, is vertical, and is along the side of the tower. It slid a bit out of place to the side, but I think it's slid back in correctly.
Yeah, I don't know what it exactly is (motherboard, graphics card, whatever). I'm obviously quite dumb with this stuff, so I appreciate the help!     

[Broni]

I decided to just vacuum all the built up dust inside

Never use vacuum inside the computer. It produces a lot of static. You might have had fried something.

[RedConverse]

Yeesh. How would I tell what exactly was fried?

[Broni]

What is computer make, and model?
Windows version?
Can you hear hard drive spinning, and fans running?
Does that sustained beep ever end?

[RedConverse]

Not sure of the make or model. The tower has 2 little labels, one that has "USitek" on it, and the other with Intel, Pentium 4.
I'm using Windows XP.
The fans are running, and I'm pretty sure the hard drive is spinning.
 Yeah, the sustained beep only lasts for a second or two.

[Broni]

Try to reseat your RAM sticks.

[RedConverse]

I'm sorry for being so clueless about this, but what are those, where would I find them, and how do I reseat them?

[squall_01]

I was really calm know I'm not, I get very frustrated when people do this first NEVER SMAKE YOUR TOWER    This an cause several problems.  First off I assume it was probably your video card that you mite have pushed loose.  Remove the screw an the card then with a light firm grip put it back in an screw it down.  Secondly you said it beeped so it could be the RAM as well.  http://ralphlosey.files.wordpress.com/2007/06/ram.jpg  This is what you RAM probably looks like.  Use the same method as for the Video Card.  But I would try the card first to see if that doesnt fix it. 

[patio]

Your vacuum could also have inhaled one or more MBoard jumpers.

[RedConverse]

Where are the MBoard jumpers, and where's the MBoard, while we're at it?

[patio]

This is a MBoard:



Manufacturing is different with each so jumpers can be located just about anywhere...