What happen if your car totalled and the insurance won’t pay for it?

What do you mean they will not pay for it [if there is coverage, they pay the cost of repair or fair market value, whichever is lower but people can never find the same car for the amount insurance pays]. They probably will refuse to pay you any car rental, but you could argue you are entitled to the loss of use from the accident to when you reasonably could find a replacement. What reason do they give? Whose insurance is it?? We need more information to be able to answer.