What happens when I hit the credit limit in my reader account?

It depends on your subscription.

If you are a customer trying out the product:

The OAG-managed reader account will be temporarily suspended when the limit is reached. You will not be able to execute any SQL queries until more credits are requested. In order to receive more credits for the trial reader account, please contact your account manager.

You will face this error message when trying to run queries when the limit is hit:
Warehouse 'GENERAL' cannot be resumed because resource monitor '{1}' has exceeded its quota.

If you are a customer on an active subscription with OAG:

The reader account will not be suspended, but we might charge overages for credits used beyond the complimentary amount to your account, according to the terms of your contract with us. If you are unsure of these details, please check with your account manager.