dell Xps L502x wont power up. Just fan runs on full boost
Unplug, remove or disconnect battery. Hold power button 30 sec.
Remove and reinstall the hard drive and memory modules.
Plug in (no battery) -- AC only. See if it'll power up. If it does, reconnect the battery and try again.
If it'll power up on AC but not with the battery installed, the battery is toast; replace it.
If it still won't power up, the mainboard is toast. Given the age of the system, repair is likely to cost as much or more than the system is worth -- replacement rather than repair may be preferable.