Why can't I log in?

Bought an ex-lease macbook last week which is like new. I do have 12 warranty, but as it's a Sunday thought I would ask here before phoning tomorrow. Earlier in 'settings', have changed password in 'user' and created a new user account. Have gone back to log in now - the user picture has changed, but it won't accept my current nor past password. I am stuck and unable to do anything. No option to change user either. Very frustrating as I've saved the school's christmas play in docs and not in my cloud!