Unable to turn computer off/ on ... Screen is white .. Mac pro
mac pro OS 9
screen frozen
Mac Pro product
Do you really have OS9 installed? I ask since the original Mac Pro (2006) has Tiger 10.4.7
Reset the SMC and PRAM
Intel-based Macs: Resetting the System Management Controller (SMC)
Try booting to Recovery (Lion and Later)
OS X: About OS X Recovery - Apple Support
Boot from Install disk