|First of all you shouldn't blame your sister.|
Now, start trouble shooting it.
Check the to the computer by using that cord to power another device. (if you use a test meter be sure you understand voltages present can kill)
Remove power plug and press on button a few times. Put power plug back in with the back switch off if it has one. Turn on switch while looking at ps fan. Does it try to turn or turn on? Any keyboard lights? Any beeps?
"Best Practices", Event viewer, host file, perfmon, antivirus, anti-spyware, Live CD's, backups, are in my top 10