What "big mistake" did you make?
If you really need to be install the OS etc then insert the DVD that came with your Mac, select Apple menu> System Preferences, and then click Startup Disk.
Select the DVD volume to startup from that disk.
From the startup disk you will have options to re-install your Mac.
If you can let us know what the issue is in more detail perhaps we can help...
Have you managed to sort out the problem?
I'm moving this thread to the MAC sub-forum (because your thread is not Logic related).
Hope you've managed to re-install your system if that was the issue.