You could have a simple little tiny memory chip that failed on the motherboard.
Or, it could be the video-related chips.
Hearing your fan running means you have power. That's about all we know.
Most likely, your data and files and everything is safe, or with very little damage if any.
I'm betting it is either a failed memory chip, a video chip, or the boot tracks on your hard drive that are no longer available for work today, one of those, probably only one. Most likely, all your files, photos, videos, etc. from your hard drive are there to be found, rescued, put on another drive, ("imaged"
or simply used again on a new machine.
Are you located somewhere where you can take it to a reliable repair location? Someone trustworthy? Someone who specializes in fixing older computers, restoring data?
They can, most likely, diagnose the problem, at least extract your hard drive, probably make a copy of the hard drive, (back-it-up). And then go on to rebuilding your system, from XP to Windows whatever.
They may want to sell you a new computer, with an image of your hard drive transferred onto the new one. And, altho that might sound like a radical step, it doesn't have to be too expensive, they could sell you a rebuilt machine with XP or whatever windows you want, a second "image" of your old hard drive data on it, and you're back in business. Cost of a reliable re-built machine can be as low as $250., plus labor.
As for not backing up your data: no excuses. On-line services, flash drives, all sorts of options available to you. A rule of thumb is once a week, but once a month is better than never, and once a day for someone that relies upon their computer each and every day of the year for work and play.
By the way, how are you posting this on DU? Do you have a second computer? Back-up files from an on-line "cloud" from your old computer can be put on this device. Seldom do people other than photographers or authors and researchers have more than 100GB of real data they need from an old computer put on a newer computer with 300-500GB of space.
You're probably going to be just fine, just out a few hundred dollars for a new motherboard, or new/rebuilt machine, and all the data you want, which, of course YOU WILL REGULARLY BACK UP this time around, right?
Some people hate to spend $$ on a new computer, some hate to spend $50-$60 a year for a an on-line backup service, yet they trade in their 4 year old car on a new one and spend thousands on the new car.
Spending more than $500 on a newer computer needs to be justified, but if the old computer is useless, why not do it? Spending $50-$60 a year on an on-line automatic back-up service, just makes sense. Never be without what you had yesterday, somewhere out there ready to get to again today.