Cylinder not firing

Thanks guys. Sorry for being kind of vague. The engine is 360, it is a 87 truck motor in my 73 Dart Sport.
The plug was black and kind of grimy with oil. I did put in a new one and started the engine with the plug out and did have spark.
I didn't think it was timing really...just spitballing. It was just so weird that I went to a car show and the thing was running great. Went to leave and it started running bad... I am going to take the valve over off and see if there is anything out of the ordinary. I am also planning on doing a compression and leak down test. I just figured I would throw it out there and see if there was an easy answer.
Thanks again!