No volume on my mac
Wow! That is a seriously vague post! Most people wouldn't even bother responding.
What are you doing when you get the message? If it is on start up, your hard drive has failed. You may be able to repair it by booting from the OS disc and run the disc repair utility.
Do you mean no volume after upgrading to Mountain Lion?
Solution: Restart your mac.
I hope it works.
Retrieving data ...