Please use code tags .
Update() is a magic method that automatically runs every frame. You shouldn’t call it explicitly like you are doing inside your OnTriggerEnter2D method. Instead, have the collision set a variable, and have Update check that variable to decide how it should act. (But remember Update will always run, whether the trigger has happened or not.)
But if that were your only problem, you’d see the timer running all the time, instead of not running at all. I think you have a problem with your curly braces {}, but it’s hard to tell because you didn’t use code tags.