Let me give all the instructions before I go off to sleep. Other
forumers can help if the moderators are off to sleep. We need to
study. 
Once confirmed that your power supply, hard disks, RAM, CD/DVD
drive is connected properly and firmly, try booting up your
computer. See if it runs.
If it runs, it could have came loose because of expansion due to
heat and contraction once you cool down the PC. This could result
in chip creep. It's not a big problem, just clean up and reinsert
them back again.
If it doesn't run, check for burns and smoke. Open up your CPU
casing and smell it if there are anything that burns. No burns and
smokes, continue to the next step.
Unplug everything, from keyboard to printer that is connected to
the CPU.
Try booting up. If the POST screen shows, power off the PC. Wait
for 30 seconds. Off all power supplies. Plug in the keyboard. Boot
up again. If there is POST screen, repeat the steps again with
another hardware. Only plug in one device back one at a time. If
you plug in one device and POST screen no longer shows, it's that
device that is giving trouble for you. If it's still under
warranty, replace it. If it's not, consider whether you want to
replace it or repair it.
After this, check your monitor. If it is a LCD, put it in a box,
bring it to your friend's house and test it. Does the POST screen
shows? If yes, it's not your monitor's problem, probably with your
own CPU. If it doesn't, replace your LCD monitor. If it is not a
LCD monitor, try sending it for repairs. CRT monitors last very
long, if not, buy a new one. It costs less than $50 for a
second-hand CRT monitor.
One more thing I forgotten, buy an additional battery and replace
it. If it starts up properly after replacing, it means your battery
is dead. If even a replacement battery doesn't help, I recommend
you buy a new motherboard already unless it is narrowed down to
some devices that are causing problems.
