If you have a damaged system, you can take out the hard drive out of your computer, put it in a enclosure. Back up your data, and install back the OS from the disc. If the hard drive is really hard to take out (e.g. 2008 imac) but has firewire, you can use firewire target disc mode. You will need a firewire cable and another mac that supports firewire too. backup your stuff again and reinstall the OS
ifixit has good tutorials on how to do such stuff.

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