Can you boot into Internet Recovery? Is it a Mac Pro (desktop) or a Macbook Pro (laptop)? Can you confirm the year/model?
You can use OS X Recovery - OS X: About OS X Recovery - Apple Support .
If FileVault was enabled then you will need to visit the nearest Apple Store, if the key was stored with Apple.
Please also see
OS X El Capitan: FileVault recovery key
OS X Yosemite: FileVault recovery key
As a further precaution, you can store your recovery key with Apple. When you choose this option, you pick three security questions. If you forget your recovery key, you can contact Apple Support, provide the answers to these questions, and retrieve your recovery key. For more information, see My Apple ID website.
One option you may want to consider, as the worst case scenario, replace the internal HDD. If a firmware password was set, please see If you lost or forgot your firmware password - Apple Support
Only Apple Retail Stores or Apple Authorized Service Providers can unlock computers protected by a firmware password.
If you can't remember the firmware password for your Mac, schedule a service appointment with an Apple Retail Store or Apple Authorized Service Provider. Bring your proof of purchase (original receipt or invoice) with you. If you plan to visit an Apple Retail Store, make a reservation (available only in some countries).