hollie tillbrook , 17 , was out with her boyfriend in basildon 's festival leisure park , essex , when she collapsed .
doctors believe her potassium levels had plummeted to such a low level that her heart completely stopped .
but luckily , two officers , believed to be from the met police , rushed to her side and gave hollie life-saving cpr until an ambulance arrived .
but the teenager then made an unexpected recovery which stunned medics .