After the women had gone, the goons went back to town and reported all this to the Cabal, who then met together to hatch a plan: they bribed the goons with wads of cash to say that they'd fallen asleep and Jesus' apprentices had stolen his body.
"If the story gets back to the governor," they told the goons, "we'll take care of it, so don't worry about any trouble."
The goons took the money and spread the story around. It's the story the Cabalists still tell today.
A good conspiracy theory has layers upon layers of intrigue. Reports of secret meetings and payoffs. Inside information and cover-ups. Circumstantial evidence of motive and opportunity. This story has it all.
Apart from this story's being in the Bible, the simple explanation for a dead body gone missing – the one any police detective, any ordinary person, would consider first – is that someone took it. No conspiracy theory necessary, but it does make a great story.
Matthew's elaborate conspiracy is a counter-story. It says more about the pressures and accusations the community was feeling a generation after the fact than about the secret meetings and cover-ups. The implication here is, ironically enough, that Jesus' followers were facing accusations that they were themselves dishonest, starting with being dishonest about what became of Jesus' dead body. It wouldn't be the last time Christians were accused of behaving badly.
Nobody can say with any certainty what actually happened to the body. Mark's story, the earliest of the Gospels, says nothing about guards and nothing about what happened after the women ran from the tomb. All we know from the earliest Christian correspondence is that he "appeared" to his followers, and they were convinced that he was alive among them.
That sense of Jesus being alive among them is really all that matters. The obsessive insistence that faith must hinge on the historical particulars about a missing body reduce the importance of the resurrection to a parochial curiosity. The power of the resurrection is exactly the opposite: faith, the actions and power and richness of life available to people seeking to be authentically human, is no longer limited to a particular person, place or time.