How can I fix my UPVC front door?

When you turn the key it just turns but the door is still locked and unable to open it. This happens from both the inside and outside. We've unscrewed the handle from the inside and this shows that some mechanism is working, but the door still won't open. Don't know what make the door is, I think it is just a standard upvc front door with 2 half skinny windows with red roses on if this helps ??