Form Over Quota error

Users are unable to complete their forms because of this error. So can you guys please explain why we get this error and what's the best way to resolve it.

Is the issue related to the forms that are shared with your account by the user nitishsporty? 

The account nitishsporty was previously on the Bronze plan but it was downgraded to the Free plan 3 days ago because we were unable to collect the payment for the account subscription and it is now overlimit. You should inform the owner of the account nitishsporty to re-upgrade the account back to the Bronze plan.