now it just crank crank pranks and it won’t start?

I changed the mass severe flow sensor next I had to change under the filter, the plug-in play car started up was learning , next thing that happened the car wouldn’t crank it would crank, but it wouldn’t turn over so I went and put a little gas in it. It turned on. I backed it up installed and now I take the negative cable off the battery so it doesn’t kill the battery sitting. I figure out what’s wrong with it. It gave no warning. It just died out and now it just crank crank pranks and it won’t start. What else could it be this