Metrics flat lining commonly happens when a website is updated as most people don't realise the impact that a small change has on the overall tracking of a website and how one element of data flows through to give life to many others.
During your onboarding the team would have set up up with with a custom Google Tag Manager that contains all of your old analytics details.
Install Google Tag Assistant in the Chrome Browser
Open it
If it is blue green or yellow data is flowing
If it is red please contact your developer.
If you are using the rocket portal the data can be retrieved by contact the CS team