Dimension 8400 tower is malfunctioning...

So I have a Dimension 8400 and today I was cleaning out the shell. I opened the shell *** out all the dust. I ensured that all of my cards were tighly secured then I closed the shell. Reattatched all the wires. Powered the tower up. Suddenly I hear this strange alarming sound coming from the tower. There is no pictue on the screen also I notice that there are four lights on the back of the tower that are now on that I have never seen before. ABCD. The A and B are like orange or amber and the C and D are green. Oh Lord... what has happened. Iam scared to find out but I need to know quickly. Please I need your help! Thanks!