Talk to a disability attorney who handles ERISA claims. Most will provide free consultations. They will want to see, most likely, your policy and correspondence from the insurance company.
You should contact an attorney who deals with ERISA disability claims as soon as possible to discuss this termination of benefits. There could be time limits and steps you must take when challenging a denial or termination of benefits under this type of disability plan. It can be very important to meet all of these requirements. The time limits can often be found in the letter terminating your benefits and/or in the plan documents/policy itself.