Doesn't turnover....

I was driving a 94 Dodge Caravan and it worked until it suddenly slowed down on the highway. I pulled over to the shoulder and the vehicle wouldn't turn over.
I checked the oil , it was 3 qts low. I had it towed home and I poured the missing qts. I also checked the spark plugs and changed 3 so far. The belts are ok,...no leaks
I had a tune up last year in June. What could be the cause that the van is not turning over to start?
alt & starter and every electrical part is working.. please help..
Stranded on the block....