Changed password now unable to access MAC Mini

First make sure you did not inadvertently include a space character at the beginning or the end of that password, which may not be readily apparent. Also try toggling Caps Lock to make sure that's not a factor either.

Then, If you forgot your Mac login password - Apple Support describes the applicable steps. If you can't reset your Mac login password describes what to do next. Follow them all the way through to their conclusion.