The first thing to check in electronics are the ABC's. Antenna's, Battery's, and connectors. I would first remove the Battery back-up from the situation and go straight to the wall. If it boots, then use a surge protector and find a new back-up battery. If not, I would suggest making sure the your power cable is completly connected, slight loose connections can cause low voltage, and alot of heat.
If you can verify that the power source is working, your next step is the power supply. Since there is no simple way to determine if the power supply is dead with knowing what exactly you are looking for, I would suggest replacing it if you can, and or bringing it to have it done.
I am wondering if your computer completes the POST. If it does and your keyboard is not working then you should get some form of error message. If this is not happening then you will need to reflash your bios.
Generally speaking though, when a computer locks up prior to being shut down, it is rarely if ever a bios issue. The bios only really directs data flow and assigns IRQ's and memory allocation for just long enough to get your OS up and running. The OS for the most part takes over after that. What we really need to know is how far into the boot process does the computer get without locking up or getting the dreaded BSOD. If youare getting a BSOD then you should make note of what the error message is. This generally determines the level at which the OS stops working.
Could a power rail on the powersupply have gone out and cause you to loose keyboard and mouse functionallity, not very likely. You would normally have a whole lot more dead items. Since USB runs off of the 5v section of the Powersupply, generally your Floppy drive, and a large majority of you peripherals will fail to come alive as well if the 5V rail is damaged. Sound cards, ethernet cards, USB, firewire products, Specialized slots, and some other standard PCI, PCI-E, or PCI-X slots may not function normally. This can and will affect the ability of the bios to post, since it needs to power to those peripherals to complete the post process.
Of course these are the simple items. Could it be a memory allocation error due to a bad stick, or or a problem with the onboard controllers, sure it could. Definately something in the southbridge side of the controller if that is the problem. It depends on what mother board chipset they are using and what processor is involved. Some companys have completely gotten away from using southbridge controllers. Just depends on the age of the actual computer and if this has occurred in the past.
Jahlall.
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