Generally, your server will be suspended because your latest invoice was unpaid. After the second invoice reminder (2 days), your server will be set to Suspended. This means that you will not be able to access your server files and your server will be offline.
To fix this, go to the Billing Portal and login to your account. If there are any unpaid invoices, pay your invoices and your servers will automatically be unsuspended. They will be available for you again on the Raw Power Panel in around 1-5 minutes.