What can you do if the offer I got from bodily injury was withdrawn? They wanted to pay me 400, I wanted 500?

I was in a car acciedent and was hit by 2 cars from behind @ 65 mph. Safeco (the other party's ins. co) wanted to settle for $400.00. I wanted $1000.00, but would settle for $500.00. They withdrew their offer. What can I do?