My Mac will not boot up and all I am getting is a grey screen. I can not run the diagnostic m

Do you hear the startup chime?

Is power light on steady?

Are any diagnostic light lit on the logic board or by the memory sticks?

Can you boot to boot manager screen? Yo do that by depress and holding the Option key during bootup.

Try resetting the SMC and NVRAM/PRAM

Intel-based Macs: Resetting the System Management Controller (SMC)

About NVRAM and PRAM

Can you boot to Recovery (lion or later only)

See if this helps:

About the screens you see when your Mac starts up - Apple Support

Last,

- Make an appointment at the Genius Bar of an Apple store.

Apple Retail Store - Genius Bar