Password not recognized after Apple servicing

After coming back from the Apple center my Mac does not recognize the password. When I try to change it through AppleID, my Mac does not manage to connect to iCloud’s server.

Tried to reset it through Cmd+R and typing resetpassword on the terminal. But I get the same mistake. The same goes if I try to update OS (currently on Sierra BTW). Tried to reset all internet connections Opt+Cmd+Shift+R+Pwr, but did not have any luck.

After 1.30h in call with Apple Support, they told me I should completely reset my machine but I would like to not lose my data.

Do you maybe have any other solution to recover the password or save my data?

Apple MacBook Pro 13" 2016

Take your MacBook Pro back to where it was worked on. Your machine should have been returned to you in usable condition.

This is, unfortunately, a lesson all should learn: back up all your data before having any work done.