1994 Oldsmobile Ciera Fuel system

I bought this car from someone who did not drive it for 2 years. I have changed the master cylinder, the alternator, gave it an oil change and tune up and it ran great for about a week. Then one day it just would not start, it had spark and I thought no fuel. I thought it was a fuel pump, so I bought one had it towed to my mechanic and asked him to find out why it wouldn't start. He called the next day and told me that it was flooded, he pulled the spark plugs and cleaned them up and put them back in and it started. We picked the car up, did not drive it the next day and it did not start. What could cause the car to flood without touching your foot to the gas pedal. I would appreciate any help or advice.
Thanks, Thor