Computer only works on its side?

[Urban]

My computer has been working fine for the l8 months. Today I turned it on, the CD light flashes but the HD light doesnt come on and my monitor isnt activated. I put it on its side to work on it and it boots fine, stand up again and nothing. I have reseated the graphics card, change the power supply and ribbon to the HD, everything seems to be plugged in fine. Please help, I am pulling my hair out!
System is P2 333 on an LX mobo, Seagate Cheetah 6.4gb HD, 128mb ram, Intel i740 8mb graphics, AWE 64 sound.

Ps its been working great up until now, just cant figure out why it only boots on its side, 100% of the time but not standing up!

[CMonster]

Probably the hard disk is motor choking or you have a loose connection somewhere that makes better contact when the box is on its side.

[Sterling]

Be sure to reseat all your connections on all your internal devices such as you Hard disk, CD-rom etc. Be sure to check the connection of the molex (power) connector to the hard drive. If the power leads were not found to be loose or weren't found fully seated then there could be a break in the wire of the power lead. Try switching out the molex lead presently on the hard drive with one that is free. If you are inclined and capable to measure voltages, you may want to verify the 12 volt and 5 volt DC leads of the power connector. The fact that you state it works when layed on its side does suggest a broken or loose lead and that laying it over is causing the problem lead to have some tension applied to permit contact to complete the path needed for the go juice.
Later.

[Urban]

I appreciate your help and will try the things you have suggested.

Cheers guys :-)