If you did not trigger a downgrade to Free Plan yourself, the following might be why your Project on a Paid Plan was downgraded to a Free Plan:
Payment failure typically happens when your existing active card listed under your Billing information is no longer valid. Learn more about payment failures here.
If you fail to update the existing active card within the 7 days grace period for payment failure, your paid plan Project will be downgraded 7 days after the date of renewal.
To continue your subscription to a paid plan Project upon renewal, you will need to have a valid and active credit card under your Billing Information.
If there is no existing active credit card detected (i.e. credit card is removed), upon subscription renewal, your paid plan Project be downgraded immediately to a Free Plan immediately, with no grace period.