Outlets and lights don't work but plug-in receptacle tester indicates all ok
A plug in electrical tester will show voltage and let you know you have power there, but it will not show amperage.  So if you have a loose wire connection or a wire break under the insulation on one of your receptacles that feed the others, you will see voltage, but if you plug a light in or something else that will draw some amperage, the voltage drops due to the amperage draw.  If you plug in your plug in voltage tester and then plug a light in to the other socket, you will see that the plug in tester shows no voltage or very little.