usually you hold down the Alt/Option-key at startup of your Mac to get the Boot Selection screen.
There you can choose which Operating System to boot into.
Once in OSX you might wanna go to System Preferences - Startup Volume and set your OSX to be the default.
If that's not working you should boot from your OSX Install DVD (holding down the C-key at startup) and use Disk Utility from it to verify/repair your harddisk/OSX.
I hope the Option key solved the problem. If not, it is possible that the OP might have not followed the proper Boot Camp loading instructions and might have wiped their MacOS partition. I have seen a few cases where people have done that... most without a current (or any) backup of their MacOS either...