Cycles no reflections in the final render

Good evening,
I am working on a model from Blendswap and right now I am texturing it. In the viewport it looks realy nice also with some reflections on the windows. But if I render this final, there aren´t reflections on the windows anymore. Also the final image is much darker than the other one. The lightning comes from a sphere with envirement texture. All elemets are visible in the render and the viewport.

I have uploaded two images.
Maybe someone can help!

Try use CPU only mode, check maybe not enabled render layers. Hard to tell looking screenshots, it very looks like environment turned black for some weird reason.

Are you sure you did not mix up final and preliminary render? It looks like you reduced the number of bounces for the final image. Go to Light Paths in Render tab and select Full Global Illumination. Just a guess without the blend file.