1999 Ford Taurus driver side front door not staying latched

Just tonight when I got into my car and pulled the door shut, the door did not stay closed, I thought it might be the cold/ice (since its a near blizzard out tonight) and I pulled it closed and tried to slam it shut, to no avail I started to look at the latch in the door. I dont know anything about cars and had no idea what I was looking at. Like I said, it's very cold outside. I have noticed the past few days that the door ajar button would light up even though all the doors were shut, but I didnt think anything of it. I appreciate any help. Thanks.