My Acer Extensa 5420, started having problems with powering on, now it will not come on at all. when you power it on the lights come on, the fan starts then it just powers back off. Nothing new installed and I have checked the power cord.have reseated the RAM and tried the BIOS reset by holding the power button in with the battery out

Clean out the cooling fan/vents, etc. and try again. If the fan isn't turning, it won't boot up.
If that doesn't work, you may need to disconnect the power supply and battery and then hold down the power button for 30-60 seconds. This will discharge stored energy from your laptop. I'm not sure of the specifics, but this happens to mine every 12-18 months or so. Once you've done that, reconnect the charger and battery and try to power up again. It should work now.
If these steps don't fix it, you have serious problems.

Your laptop problem is cooling fan. Please open the laptop & clean the dust over the fan. Otherwise your fan must be replaced by new one..

Remove the battery and unplug the power cord for 5 minutes, put in the either the battery or the powercord and power on.

if it doesn't power on, make sure the charging light is lit on the front of the laptop when the power adapter is inserted and plugged in properly.

light blinking when you power on means battery low / no power.

If the problem doesn't go away you need to contact acer for warranty service if you are outside warranty contact a local repairer.

you have a brand new hard drive... probably cable is not fitted properly and the memory contacts needs cleaning. try it... good luck.

open the laptop and clean fan and heatsink... thats it

your bios in problem,some hardware can't recognize
go into nearest customer service center

Okay, Here's the deal..... I recently had the same problems with my friends' Acer Extensa laptop and here's what I did to fix it. I narrowed the issue down to two culprits 1. Overheating, 2. Defective Motherboard. I have fixed the problem but I takes time, patience and some technical ability which may not be available to some. :( (Swapping out the motherboard MAY fix the problem but if the defect is in the manufacturing process then the problems could return). Here's how I fixed it... While in the process of disassembly I noticed a small electrolytic capacitor that had seemingly broken off and had become wedged between the frame (<ital>very lucky! this gave me the clue I was looking for!</ital>). This made me think that it had broken off the motherboard. I proceeded to disassemble the laptop and found the location of the missing capacitor (there are two located right next to each other on the board) and was able to re solder the capacitor back onto the motherboard (could it be a manufacturing processing defect thats causing these to break loose? ). Mine was broken off due to what looked like bad solder joints. Your capacitors could still be attached but have weak solder connections. Also while i had access to the motherboard, I used the opportunity to clean and re apply the heat sink compound to the CPU and thoroughly clean the fan vents. I then reassembled the laptop and EVERYTHING has been running fine now for the last 3 days. Total time was about 4hrs and it cost NOTHING to repair. GOOD LUCK :)

When you say "crash" what exactly do you mean? Please describe the symptoms a bit more so I can offer appropriate help.

Just for the heck of it, try blowing out the cooling fan with some compressed air. If you have never cleaned it and the fan stops turning, the computer will go into a safe mode and not start up.
Ther should be a vent somewhere on the side of the laptop that vents hot air cooling the CPU. That is where you need to blow it out.
Hope it helps- just a longshot...

its power button doe not work please change it as soon as possible