Missing Operating system PLease help!

I recently tried to install windows on my mac using bootcamp. During installation it froze so I force shutdown my mac. When i booted it up it went back to windows but wouldnt let me finish installing. I found out if you hold down option key you can boot back into mac. I did that and messed around with partitioning in disk utility. I restarted my mac and now it goes to a grey screen then to a black screen and just says missing operating system. Please help..

It sounds like you may have inadvertently did something with Disk Utility to cause your Mac to not boot properly.

The first thing to try is a PRAM reset. Do this:

Shut down the computer.

Locate the following keys on the keyboard: Command, Option, P, and R.

You will need to hold these keys down simultaneously.

Turn on the computer.

Press and hold the Command-Option-P-R keys. You must press this key combination before the gray screen appears.

Hold the keys down until the computer restarts and you hear the startup sound for the second time.

Release the keys.

If this doesn't work to get your Mac back to booting properly, you may have to reinstall. We also need to know the year and model of your Mac and which version of OS X it's running.

Okay I did this and it now instead of missing operating system it says "press any key to boot from CD or DVD.....missing operating system" ..?

You're going to need to reinstall OS X. Do you have your original install DVD that came with your Mac? If you're using Lion you won't have an install DVD. That's why we asked which version of OS X you're running and the year - model Mac that you have.

Yes I was running Lion. I got my in Dec 2010.

Reboot your machine by holding down the Command + R keys. That will boot it to the Recovery partition. From there select "Reinstall Lion". Make sure you're connected to the internet and running off AC power not battery. Lion will reinstall over itself preserving all your applications and documents.

You may have to do some Disk Utility work first to get rid of the bootcamp partition if it still exists. So make sure you check that first. If you have to mess with the partitioning, that might destroy your data. If you have no data to preserve, it might be best to just erase the disk and start over. But that's up to you.

Thank you so much. Its installing lion right now

Missing operating system

Thanks for your help! I change wrongly the booting partition but with your advice I managed to recover the correct booting partition!