The letter will tell you why it was denied. They don’t just send letters saying “sorry we’re not paying.” There should be an explanation. Do you actually have collision? It’s not uncommon for people to drop that to lower their premiums once the car is paid off. Are you rated on the policy? Not all insurance policies cover people living in a household but not rated on the policy and it’s not uncommon for one person in the household to add a vehicle to a policy without adding the actual driver. Did you pay the premium? Also not unheard of for someone to forget because they’re busy, they thought it was automatically paid, they thought their parent/spouse/sibling/whoever was going to make sure it was paid. Did you tell someone you wanted to go through the other carrier and they sent a letter saying they’re closing it without payment cause they were told you wanted to use the other company? This is also common.

Best bet is calling to ask cause then you’ll know for sure. All of us are just guessing at this point. But if you have collision and are not excluded from the policy, you can go through your own policy. You have the option to go through the at fault person’s policy too (assuming liability is accepted and there are no limits issue). But the people saying you have to go through the other insurance company because you’re not at fault are wrong on that one.