A yellow triangle with exclamation mark shows on macintosh hard drive .

A yellow triangle with exclamation mark shows on macintosh hard drive and I can't reinstall os on it.
Please notice:
I have MAC PRO machine.
OS is 10.7.3 .

Chances are very high that you are attempting to install OS X from gray discs intended for a different model of machine, or pirated homeburns or something like that.

It's also possible that you're trying to install an OS that came out BEFORE the machine you're working with there was made. That's also not going to work out (some details about your machine would have been super helpful here, obviously)