1995 Chevrolet Lumina no crank no start

3 days ago I went out for a delivery for my job and due to the area I turned the car off and locked the doors. After my delivery I went back to the car to start it and it wouldn't start. When I put the key in and turn it The lights and everything else still functions like usual but it just won't start nor crank. I replaced the starter but still wouldn't start. What could it possibly be. If not the starter all I can think is that the engine locked up. A week prior to the issue, I went in for a oil change and they used a different type of oil supposedly better for my car. While under the car the first think I seen is on every bolt on the oil pan its oil them tripping. I checked the oil levels on it also and its beyond full. Can that be the cause of this problem. I really need help. I'm in the process of losing my job because of this problem.