What happens if you settle out of court for an auto accident and the insurance company doesn't pay?

Part of settling your claim is signing a settlement agreement.  Hopefully as part of your settlement, you have signed a settlement agreement, which sets forth in writing the terms of your settlement.  Your settlement agreement is now a valid contract which you can legally enforce against the insurance company if they have not made payment under your agreement. Your question did not address how long it has been since you agreed to settle.  Depending upon the insurance company and the amount of the settlement, it can take some time, maybe a few weeks, to receive payment.  I would contact the company and find out what is the hold up and when you can expect payment.