If the car insurance was cancelled by the insurance company due to non payment then yes you can usually reinstate the car insurance policy through the insurance company by making payment and contacting the car insurance company.
If they cancelled your car insurance policy for some other reason then you may or may not be able to get them to reinstate your car insurance and may have to look for car insurance through another insurance company.
If you just cancelled your car insurance policy for some reason and then want to reinstate the car insurance then all you have to do is tell them you want to reinstate your car insurance policy and pay the money to the insurance company.