If you can, pull the hard drives and put them in another PC that has an OS as secondary drives. Use the other PC to read the hard drives, Get the data off first. If you can get the data, great you are about half way there.
If you have another running DC, and you have an old backup of your crashed DC, restore the crashed DC and let the other DC write fresh Active Directory to it when it comes up.
If you have only another running DC, consider siezing the FSMO roles held by your "Crashed" DC (if any) and rebuild the bad DC.
If you do not have another running DC, but you have a backup, then Restore the backup and update your active directory manually.
If you don't have any of these... Blame the person who failed to plan for this emergency, consider drinking something strong, and cry alot because you may be rebuilding your domain from scratch.
When the DC is up and running again, restore your data that you saved earlier but do not include system restore data unless you are willing to take a chance on having to repeat this process.
A+/MCP/MCSE/MCDBA