How to repair/recover Snow Leopard when the installation DVD has been lost?
If you computer not booting is the reason you need your Snow Leopard disc, there are several things to try and troubleshoot your issue.


Boot into single user mode (Cmd + S @ POST (chime)).  Once the command prompt comes up, type fsck -fy.  This will attempt to repair your disk.
If this is okay, hold down Shift at POST and see if it boots.  This disables kexts, fonts, etc.
If this does not work try verbose mode (Cmd + V @ POST) and see where your computer stops loading at and troubleshoot from there.


Good luck.