The first step is to not panic it is a stressful situation, but it can be fixed with some patience. Now since you don't have a heat gun it's time to improvise use a hair dryer instead don't have one then get some isopropyl alcohol and soak a small piece of the towel and use the alcohol to break down the adhesive by simply dabbing it on the piece. If you do have a hair dryer, then make sure you have something to pry and pick at the remaining pieces of screen. Hope this helps best of luck!