Insurance companies vary on their guidelines regarding vehicles and what they will and will not insure. It is unfortunate that with your current insurance company that the rules either changed or it took underwriting time to recognize that your vehicle had excessive mileage in their opinion.
Generally you should be able to find a new company to insure you. Shop around with us for a quote. If you need more information on different insurance rating guidelines you can contact your state's insurance regulatory body.
How to Maintain High-Mileage Vehicles is an article that can help you take care of your vehicle.