help! i cannot switch off my computer!

[27 houdini's]

my computer refuses to shut down. what can i do?

the only way to switch it off is to hold down the power key for a long time

anyone any clues?

using the apple menu doesn't work either

 

[ShadowTech]

Uhhh, did you try holding the power button on the tower? That works for me.

 

[michaelsanford]

When you say it doesn't shut down what happens, exactly ?

In Mac OS X Jaguar and later you should get a grey or blue screen with a turning gear, after the Finder quits, that stays until the power it cut automatically.

When you select Apple Menu -> Shut Down, what happens, specifically ?