HP Pavilion 523n will not turn on

have you added any hardware recently or opened the case for any reason?  the power supply is often the first thing to go on computers.  if it is still under warranty, you might need to send it off.
if you want to try it yourself, i'd say open your case and make sure the power cable to the motherboard is connected securely (there should be a 20 pin connector and possibly a separate 4 pin connector).  of course, make sure the power chord is connected securly to the back of the computer.  make sure the psu is on 115 instead of 220 (unless you need 220).
good luck!