Nissan micra not starting garage don't know what's wrong with it?

I have a T reg Nissan micra. Last tuesday I tried to start it in the morning and it wouldn't start but then started first time in the afternoon. Wednesday morning it wouldn't start again and I had to get it towed to the garage. The garage couldn't find anything wrong with it at first but then it wouldn't start for them and so they hooked up a computer and it was saying there was no fault. In the end they replaced the lambda sensor and gave it back to me. It drove fine all the way home but then wouldn't start this morning. I checked under the hood myself and both the oil and water was under the min line so I've filled them both up. It still won't start but is now coming up with an oil light before I start it. Any ideas? I'm taking it back to the garage when I can get it started again. When I say it won't start the car turns over but the engine doesn't fire up. The battery is working fine. The guy who towed it said there was no spark and I did tell the garage that and they must of ruled it out. Not sure if I have any faith in the garage if they didn't even check oil and water level. Any help would be great.

Maria
May 2010