dell dimension 3100

Have you pressed either F2 or F12, and if so, what was the result?  If F2 actually takes you to system setup (BIOS) as it is supposed to do, you might want to check to see if it reports the model of your hard drive correctly.  If it does, escape out of system setup, allow the computer to try booting again, and press F12.  If you get the boot menu, try the diagnostics to see if you get any failure notices.

The symptom you are seeing can result from a hard drive failure, and may just be coincidence rather than the result of anything your daughter did.

i have pressed both f2 &  F12 and even F8 nothing just goes to a blank screen tried even to boot from my own xp disk nothing happens with it

Does the computer seem to execute the self-test normally?  Are any of the diagnostic LEDs on the front panel remaining lit after the self-test has completed?

F2 and F12 should operate regardless of the state of the hard drive since these functions are controlled by the BIOS.  Pressing F2 during the self test should bring up system setup, while pressing F12 during that time should bring up a boot menu in the upper right of the screen.  In order to boot from the Windows XP reinstallation CD you need to either use F2 for system setup so that you can navigate to the boot order menu item and set the CD ROM drive to be first in the order, or select DVD/CD ROM drive from the F12 menu while the reinstallation CD is in the drive.  Using either of these approaches should get you a message telling you to press any key to boot from the CD.  If you don't press a key within about ten seconds, the system will ignore the CD ROM drive and attempt to boot from the next available boot device, usually the hard drive.