My computer won't start on

[I am dumb]

My computer won't start up?
I am using a Dell Optiplex 6x620 computer (windows Xp), and apparantley it won't start up. I do not know why? Yesterday it was fine. The next day i tried to start up the computer, but it won't start up. When I pressed the power button, the loading bar loaded 100% and then after that the screen just turns black with a white typing cursor flashing on the top right side of the screen. The fans in the computer are moving faster than usual and the noise inside the hard drive seems to be a bit louder. So what should i do?
16 minutes ago - 4 days left to answer.
Additional Details
And also when I was on the loading screen it said either to press f2 or f12 but when i pressed it nothing happened and I can't get it on safe mode. And of course after the loading was complete it just stopped at black screen with typing cursor flashing.
5 minutes ago

[caperjack]

hope you got your winxp cd ,
you need your winxp cd ,boot to, don't reinstall winxp just go to the Recovery console ,by hitting "R" when you get to the recovery console command prompt,you want to run check disk ,to do so type in "chkdsk /r" without the ""

Microsoft how to on using and installing the recovery console .
http://support.microsoft.com/kb/307654