First, you have plenty of time to pay your bill — 20 days to be exact (the invoice is issued on the 1st day of the month following the billable month and you have until the 20th to pay).
If you fail to pay, your Commercial account will be downgraded to the Free account with the respective reduction in the number of available entities (channels, devices, streams) and platform resources (API calls, traffic, storage, etc.). The entities exceeding the Free account limitations will be blocked, not deleted. You will also see the following indication in the flespi panel:
Yes, you will lose the data for the period of blockage. Channels and devices will stop receiving new messages, streams will stop forwarding data to target platforms, etc.
The storage for channels, devices, and streams will also be reduced, so any stored data exceeding the Free plan limitations will be deleted.
Important: you will not know exactly which specific entities will fall beyond the Free account limitations and will consequently be blocked.
Once you pay the bill, all items in your account will be unblocked automatically.
If you decide to remain on the Free plan after the downgrade, you should follow the proper downgrade procedure.