If you are on a prepaid plan, it will immediately stop working until you are able to top-up your account again and re-start your 30 day plan.

If you are on a post-paid plan, I think service will continue to work, but you will receive some late payment charges. Do it more than a couple times in a row, and it could have a negative hit on your credit score.

You should dial #227 to set up a payment arrangement. https://www.koodomobile.com/en/help/past-due-bill