BOOT PROBLEMS, HARDWARE VIEW
This is a general outline as to why machines have boot problems.
Usually if the system tries to boot, i.e. you actually see the OS's OEM logo (Windows,etc.) it's usually a safe bet that the problem is software related. At that point the system, OS included, at least has access to the hard drive.
POWER SUPPLY UNIT or PSU
The PSU is designed to provide needed power to the hardware in any given system, it is NOT designed to prevent nor protect the system against electrical surges and/or spikes. If you own your own home you should have a spike arrestor installed, it's a relatively inexpensive device that only costs about $25 -$30. If you're not comfortable about how to install the device then have a qualified electrician do so, the cost should not be much more than the device itself. Secondly, or if you're in an apartment or you lease, get a computer grade power strip that also provides protection for your modem. Better yet, get a UPS (universal power system), that provides the function of a power strip, protects against surges/spikes, and provides emergency power to safely power down the system in case of a general power failure. Most of that should be a no brainer but it's often a simple step that is frequently overlooked.
Everything in and connected to the system depends on a fully functional PSU, if it fails or is simply overrun by a power surge/spike everything is open to damage. Such includes but is not limited to BIOS, CPU, RAM, etc.
A functional PSU fan can give a false positive as it operates on 12vDC.
As noted elsewhere one or more of the system board's power connectors HAS to be 12vAC hence the name switching power supply.
The system board usually must be grounded to the frame, don't assume that the ground cables from the PSU are doing the job adequately. Sometimes power switches located on the front of the machine (off the cable pigtail) have a ground lead that must be grounded to the frame.
Short of the above PSU notations there's not much to go wrong with the BIOS. Various problems that arise with hardware usually are prompted by an improper setting and most likely at worst you'll suffer the consequences due inadequate device support from the BIOS. Without the BIOS everything short of the power supply is so much dead weight or scrap metal, etc.
CENTRAL PROCESSING UNIT OR CPU
With a defective CPU it's possible that the system will light up but will fall far short of completing POST. CPU problems arise from power spikes/surges and over heating.
RANDOM ACCESS MEMORY OR RAM
In nearly 15 years of trouble shooting systems I've yet to actually see defective RAM short of obvious physical damage. When tested on a testing machine, suspect RAM was proved to be functional.
HARD DISK AND FLOPPY DRIVES
Improper cable connections and the wrong jumper settings are the misused rule here. Where the ribbon cable is connected (upsided down) the system will give an error and probably will not boot, there's usually no permanent damage as applied voltage is going direct to ground resulting in no connection. Improper power cable connections are sometimes possible with a great deal of applied pressure, everything will be fine, that is until you power up the system, at that time your HD, etc. is DOA. The good news is that your data is still safely on the drive's platter(s) but getting access to them requires the use of data recovery technicians, and they aren't cheap, you really gotta want the data bad at that point. Better to backup, backup, backup, etc. and preferably to a removable media.
A defective BIOS will cause translation problems with the HD, etc.
As noted above this is a general outline based upon 15 years of the "School of Hard Knocks". Trained technicians (hardware and software) usually aren't taught customer relations so their people communication skills fall short because their conversation is full "techno speak" that only confuses further the average end user and, as much as they might like to, they can't be there to help with your specific problem. Engineers usually have better people skills but usually access to them is limited or restricted. This can be a pain because when the machine has a problem it's usually after business hours or on the weekend when technical help is not readily available.
Hardware, new and used, does go bad, that however is the exception rather than the rule.
If you have a problem, please use the applicable forum subheading or your best guess.
BOOT PROBLEMS, HARDWARE VIEW