If you are stuck in a particular quest and see no options to go further, you can always go to Play / Play Campaign and press the Advanced key (indicated below the window with your save files). Then, choose a quest before the one the issue occurred in and see if you can continue from there.
In addition, should you need assistance with a given quest, try consulting the official Dying Light Strategy Guide
released by Prima or a similar walkthrough found on the net along with a
number of YouTube guides released by the community...