Help my Compaq computer wont turn on.?

When i try to turn on my Compaq 7600 desktop all i hear is the hard drive turn on and make its normal sound and the CD drive works opens/closes but for some reason when i turn it on the screen is blank theres nothing at all on the screen not even a glow and another thing is the fan on the back of the computer is... show more When i try to turn on my Compaq 7600 desktop all i hear is the hard drive turn on and make its normal sound and the CD drive works opens/closes but for some reason when i turn it on the screen is blank theres nothing at all on the screen not even a glow and another thing is the fan on the back of the computer is very loud i never heard it so loud before i also cleaned out all the dust on the inside but still it wont turn on.I even tested the monitor on my new computer but the screen says cannot connect or something like that.Please help:(
Update: There is no sound coming from the speaker at all.
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