If you choose not to upgrade after your 14-day trial period, your account will be downgraded to the "Free" plan. When your account is downgraded, you'll be subject to the limits set in place by the free account. No data will be removed during the downgrade, but if your records exceed the free limits (300 contacts, companies, etc.), then you won't be able to add any additional contacts without upgrading. In addition, if you have more than the free limit of one user, any additional extra users will be temporarily disabled. All integrations, email, and files will be disabled.
You can upgrade your account at any point, which will restore access to your users and enable you to add additional contacts.