Embraer ERJ 145 First Officer reported a loss off pressurization control during initial climb out. A level off was initiated followed by a successful approach and landing at the departure airport.

Narrative

While operating Aircraft X to ZZZ1 out of ZZZ; we experience a complete loss of pressurization in the cabin. During all preflight; setup and before takeoff checks; pressurization indications were normal and no abnormalities were observed. Upon the initial climb out; the climb check was completely and pressurization indications were normal with an increase in delta P and cabin altitude staying steady. Climbing through 9000 ft; cabin altitude started climbing rapidly with delta P dropping to zero. A level off was initiated from the CA and the QRH was referenced. The issue was reported to ATC and crew communication was established. Following the QRH; pressurization was reverted to manual mode where indications initially stabilized. Cabin altitude momentarily climbed above approximately 10000 ft causing the cabin altitude alerter to go off. Crew oxygen masks were donned for the rest of the flight. Passenger oxygen masks never deployed. Manual pressurization did not work. The entirety of the QRH was completed. Due to pressurization being completely lost both in auto and manual mode; the crew decided to return back to ZZZ. A descent lower was immediately initiated with the cruise; descent; approach and landing checklists completed before arriving back into ZZZ with no incident.