I fell through the floor while exiting one of the large tower blocks near electro. I had used the ladder to go up and search the flats, but when i used the stairs i got stuck. Basically i came to the bottom of the stairs which are broken, and tried to hop off of them onto the rubble. Thats how i fell through the floor. I am now stuck in the "basement" area. I can't get out because there are invisible walls keeping me there.