2000 Ford Mustang Service engine light

Its been really cold lately down to about -35 in the mornings and my service engine light just came on this morning. My tank was on E so I ran down to the gas station and then after putting some in the light came on. So I went down and filled it up again and resealed the lid to see if that had any effect on it but nothing changed. I don't really think the cold had anything to do with it but you never know. I hadn't noticed anything wrong with it until today when I was driving it around. The car has been fine ever since I bought it and has run great. But today I noticed a loud poping or crackling sound that sounded like it was comeing from the right tail pipe or possibly under the hood even and I don't know what I could have done to get that sound to come from it. I thought that it could be a spark plug. That some snow could have melted and built up moisture and ruined one but when I drive it it doesn't feel like its missing. If I drive over around 60 mph the service engine light starts flashing. I was wondering what could possibly cause this to happen after it was fine last night and how much it would run me to fix it thanks