Generally, your server will be suspended because you didn't pay your latest invoice. After the second invoice reminder (2 days) your server will be set to Suspended. This will mean that you are not 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 and available for you again on the Raw Power Panel. This generally takes around 1-5 minutes.