Unplug the system, remove the battery and hold the power button for 30 sec.  Remove and reinstall the memory modules.  Try powering up on  AC only -- if it will, but won't with the battery installed, the battery is bad and will need replacement. If it still won't power up, remove the hard drive.  If that allows it to power up to a BIOS screen (F2 at powerup), replace the hard drive with a new one and reload the OS. If you still get nothing, the mainboard is bad;  in all honesty, on an eight year old system, start shopping for a new system - this one isn't worth a few hundred dollar repair.
My Dell 5491 will not boot or go to the logo screen. I have done the following: - Disconnected battery and held down power button for 60 sec. - Removed all RAM - AC adapter is working. Tried a working adapter with same results - Fan does come on for a few seconds. - Power indicator and keyboard lights come on for a few seconds. As a side note (don't think it's relative?) before it stopped working I had set up the win fingerprint reader. If someone could point me in the right direction I'd appreciate it. Web was of no help.  
Dell 5491Does not go to the Dell logo