Your dad might have accidentally interfered with the wires connecting the system to the speakers or the audio settings. However, the only way you can establish exactly what is wrong is by having a qualified computer technician take a look at it.
Restart your computer, see if the sound comes back. If not, check to be sure the speakers are connected & the volume is on. If that doesn't help are, check the sound card to be sure that's properly connected. If none of that solves the problem, have it checked out by someone who knows what they're doing. Many yrs. ago, Circuit City checked my computer & installed a new sound card for free... well, I did have to pay for the sound card... :)