Have Apple or an Apple Authorized Service Provider examine the laptop to provide you with a repair estimate. While there is a free Apple keyboard repair program I'm not sure your issue will be covered by it since the whole keyboard is not functioning. In the mean time you can use a wired USB keyboard.
Make sure to have a good backup before having the laptop examined or repaired.
Here is information on the free Apple keyboard repair program (only valid for up to four years after purchase):
https://support.apple.com/keyboard-service-program-for-mac-notebooks
FYI, here is the current list of the publicly acknowledged free repair programs for various Apple products (some products such as your laptop may have multiple programs available):
https://support.apple.com/service-programs
Edit: Try restarting the laptop. Disconnect all external devices from the laptop. See if you can perform an SMC reset using the internal keyboard (must use the internal keyboard). Try a PRAM Reset (may use the external USB keyboard -- try holding the reset for at least two chimes if possible).