Hi, I'm Ashik. Welcome to JustAnswer. I'm reviewing your question now, and will post back with your reply ASAP
I need some information from you, so I can help you better.
What version of Mac OS X are you using ?
What happens when you turn on your Mac ?
By start disc, do you mean the installation DVD came with your Mac ?
How did you load Windows 7 on it ? Using Bootcamp ?
Bootcamp won't touch the OS X start up disk. So it should be still there. Turn off your Mac by holding down the Power button for 5-10 seconds. Then turn it back on and immediately hold down Option key on your keyboard and this will show start up disks. Select Macintosh HD or your Mac's start up disk and press return. This will boot into OS X desktop.
Then Bootcamp and restore the partition. Then try to install Windows again if needed.
Please let me know if that didn't work, so I will continue to help you
You're most welcome :)