1998 Mercury Sable My car won't start

About 2 weeks ago, my car started lagging, like every time I turned it on, it would take a second or two more to start. I didn't think anything of it until one morning it didn't turn over. I got it jumped, thinking it was the battery, and it worked. This happened a few more times in a week's period, jumping it worked every time. Then a few days ago, jumping it DIDN'T work. But I let it sit for a few hours and it turned right on like nothing was ever wrong, this happened for a few days as well. It didn't start, let it sit, then it turns on. I started calling places and asking around, and the majority said it might be the spark plugs, so I had a guy (who said he knew what he was doing) change the spark plugs and wires. Now, it doesn't turn on, doesn't turn over and has sat for 3 days, and still doesn't do ANYTHING! I need help. I am a single mom, only have this car and need it to work. This is such a sad way to start off the new year. Please any help or suggestions would help.