When a limit is reached, what happens to the submissions / forms?

2. When a limit is reached, what happens to the submissions / forms.

If you reach any of your limits, you will access to your account and submissions, you could download them and still edit your forms too. 

Once you get notified you will be able to upgrade your account and avoid to get your forms disabled due to overlimit status, your account should back to normal if you either upgrade or if the month has already passed (limits reset the first day of each month US Eastern Standard Time). 

The only limit that will affect your submissions is the storage, free accounts are allowed to store only 500 submissions on their accounts, if this limit is passed then the oldest submissions will be deleted, but they could be recovered by upgrading at least to the Storage plan.