Oil Cooler Problem

I had a problem and removed my oil cooler to fix it ,after refiting the oil cooler and of cousre  replacing the oil and filters the engine wont start.Does anybody know why?I suspect an oil pressure sensor but really dont want to start trouble shooting untill i either get an idea from this forum or find a person more qualified than myself to talk to.
If any one has a clue i would  sure be happy to hear it.
Thank You
Grahame