1998 Mercury Tracer battery or starter

I'm not sure what the problem is really. I'm not stupid about cars just really stumped. The car has been driving fine until last night. drove all day with turning the engine on and off and it started back just fine then last night went to friends to do some work and went to leave and there was nothing no juice what so ever in the battery. tried to jump it off using her car and it still didnt want to start. i had her take me to my house and i got my battery charger wich also runs alternator tests and battery tests. charged it on a 2amp trickle charge and it still will not start. am not for sure but i believe its the battery but am also told that it could also be the starter. is there anything else on this particular car that could also cause this problem? any answer would be greatly appreciated.

thank you

patrick