My Mac will no longer boot!

You can try to boot to the OS X DVD (put the disc in the drive and hold down C at the startup chime). Run Disk Utility and try 'Repair Disk Permissions' on your Macintosh HD.

That might do it if it is a matter of permissions you've changed.

 

My Mac will no longer boot!

The same thing happened to me. I was ( still am) a newbie. I had owned the mac for less than a month. It would just boot, then loop back and restart everytime. I wasn't very versed in the commands (chmod, chown, etc)at the time.

I eventually did a re-install but I chose the option to backup my current install to a previous install folder. This worked and got my system booting again. All of my applications were still there, however some needed to be dragged from the dmg file to the applications folder to work properly. Once this was done, I could simply copy the pref file from the previous installation and all my settings were restored. This might not work for everything, but maybe can save you a little time. Who knows, maybe someone here can give you a way to restore the system without having to re-install.

I had bootcamp as well. I could boot into Windows ok. Then I manually backed up my crucial files from the machintoshHD before doing a re-install.

I booted from the install DVD and ran repair permissions, but this did not solve my problem.

Best of Luck to you.