kingbee

I have a series 5 fc, with a series 6 13B turbo extend port. I had the engine rebuilt about 4000 kms ago and had 3mm seals put in too. My problem is this, after driving it perfectly the day before, the next day i topped it up with a bit of oil, then went to start it and it just made the ignition cranking noise, it sounded a bit down but will not start at all. Its gone from being perfect one day to not even starting the next, like a lot of rotors will do. Can someone please give me some insight in what it could be, before i go spend thousands at a mechanic??????????