Car turned off when I press the brake now no power at all battery fully charged on 2003 Pontiac Grand Am

I turned it on to leave pressed the brake to go in reverse it completely shut off . Then I went to turn it back on and no power tried to pop the trunk no electrical power .

Asked by for the 2003 Pontiac Grand Am
Clean and tighten ALL battery cables to start with! I think there are two going to the positive term. and they do give some problems.