chrysler 2003

hi i have chrystler town country 2003 and i was driving and it shut off by itself and wont turn back on and the light inside wont come on as well as the radio and i took it to a mechanic they told it was he ECU so i bought a new one ans when it was pluged in the car started runin again and the we turned it off and tryed to restarded and it wont restart at all again what do you think can cause this problemm plamm i need help despretly