s7 1200 nil ptr

Hi can anybody advise
i have been doing a program using s7 1200 (first one)and have been downloading testing modyfy for a few days all ok.Now it will not go into run when i try to put it in run the fault light flashes.in the diagnostics buffer it says nil ptr in ob1??? any ideas what this means and how to sort it

Does the program compile ok?

Usually that is caused by an un-initialized pointer.