If an invoice remains unpaid for too long a Pingdom account will eventually become suspended. Currently the limit is 15 days from the invoice date for all customers without invoice payment activated. For customers with invoice payment activated the limit is 40 days (30 days to pay the invoice with a 10 day grace period).
Automated emails warning about suspension is sent out to prevent this from going unnoticed, but if it should go past the due date the Pingdom account will become suspended. All settings will remained saved in the account but monitoring and alerting will stop.
You can view any unpaid invoices under Settings->Invoices, if you have no unpaid invoices but the account is still suspended you will see that the last invoice was Credited (not paid).
If you can't see an automated way to re-activate the account in my Pingdom, just contact us and we'll help out.