You look to your uninsured or underinsured insurance coverage.
If the other driver, the one that's judged at fault for causing the accident, does not have insurance, you would look to your own insurance policy for recovery. The coverage that's implicated here is called uninsured motorist coverage. It's first party insurance. You pay for it, you pay for the premiums, and it's one that is implicated in precisely this situation. A driver- at fault doesn't have insurance, hits you and causes damage. You then recover from your own policy. If that other driver has insurance but not enough to fully compensate you, you would then look to your own underinsured motorist coverage under your policy.