Remove AC, then the battery, press and hold the power button for a full minute (this is noted in this forum's sticky!)
Battery back in, AC, count to 10 and try again.
If fail, measure the CMOS BATTERY and replace if need be. If good, then tthe next part is usually the motherboard. Sadly this may cost as much as the entire machine.
While working properly, my 2.5 years old Acer Aspire 5920G just cut its power (I wasn't running any major/massive software at that moment; only Winamp, Firefox and XFire) and I can't run it after that incident. A lappy cooler was intact and it wasn't that hot.
Symptoms include powering up after pressing power button, starting up the comp regularly for like 3-5 seconds, running the fan during the time, reading the DVD-rom and then shutting itself off. After shutting itself, it immediately repowers itself and tries to start up again, now fan DOES NOT RUN, tries to read DVD-rom again then cutting the power and retrying start up. It fails, fails and fails constantly, I don't even do anything. During the whole process I can see nothing on the screen (might be because I can never get past the first 3 seconds of startup, but I think it might be related to graphics card). If I plug in an USB lappy cooler it works when computer runs and doesn't work when computer shuts itself off, obviously.
I did try to switch RAM and ram slots, it's not the reason. It might be the mainboard, but as I mentioned it's not hot or anything, I don't think it's overheating (although I may have BURNED it, but I don't think so, well, it doesn't smell like burnt cable, and it was rather cold when it first crashed on me); 5 seconds are not enough for it to get that hot to reboot itself (although, it never boots so, I don't think I can call it a "reboot"). Using a lappy cooler doesn't contribute to the time intervals in any way. Some people on other forums say it's because of RTC battery (it's either misplaced or damaged somehow), but I couldn't FIND it (I only found a battery just behind the keyboard but I don't think it's the RTC battery; it's too small and inconveniently placed).
I have no idea what to do, not to mention no bucks left to buy another lappy for my school so... I would appreciate ANY help to bring it back to life D: