Any error codes or messages on the dash or idrive screen? Could be any number of things so you'd be shooting in the dark without some point of reference. Best thing to do is first get it hooked up to a diagnostic system.
In the absence of any firm evidence to point you in a particular direction perhaps you might want to check the fuse panel in the boot...it powers the fuel pump for example which presents exactly as you have described when not powered, ie the engine cranks over but will not fire. That's just one example of many possible reasons for the fault which will be near impossible to trace without getting it diagnosed.