Recently my hard disk stuffed up (on my pentium 3 550mhz) so i changed it with another hard disk from a pentium 2.
i plugged that in and it worked fine. I then decided to take the cdrom and floppy from the p2 and plugged it in to the p3.
i also changed cases.
i used the cables from the p2 and found the screen would not turn on when i turned the comp on.
i changed the ide cables to the ones for the p3. It worked this time but only with the floppy detached.
when i plugged the floppy in, the screen wouldn't turn on again. and now even unplugging the floppy, my screen won't turn on.

what could be the reason why the screen doesn't turn on?
could it be something to do with power?
or using the wrong cables which stuffed up my motherboard?
or the floppy or cdrom?

Please help cause i'm a newbie at computers.

(i moved the motherboard back to its original case and plugged everything in the way it was originally when it did work but it still doesn't work now.)

Sounds to me like it isn't POSTing, try removing everything except one stick of RAM, CPU, floppy, video card, and keyboard, if it comes on, try adding one device back till it fails again.