What does the conch's fate in Lord Of The Flies symbolize?

The destruction of the conch symbolizes the end of rules and of order. It had always been a symbol of civilized behaviour and became a token that there existed some form of order on the island. It was the instrument which summoned all the boys to meetings where behavioural norms and other important aspects to ensure the boys' survival on the island were discussed. Furthermore, it was a guarantee that the rules of decency and respect would be adhered to, for the one who had the conch also had the right to speak.

The conch, in being used to summon meetings, also made it possible for the boys to socialize and discuss their concerns as well as their fears but also to consider a means to make themselves noticeable so that they could be rescued. It became a source of comfort for especially the younger and weaker boys, such as the littluns and Piggy who would otherwise not have been listened to. In this sense, it gave them power.

For these reasons, the conch became the most powerful object on the island. Its deep, booming sound created an awareness amongst the boys that something important was about to happen. It is ironic, though, that such an unsophisticated object could attain such a great measure of significance. Further irony lies in the fact that, because it was so simple and natural, it would fit in better in a savage environment, rather than one in which it became a device to maintain civilized order, and, therefore, be an emblem of sophistication. 

Once the conch was destroyed, everything fell apart almost from the moment of its destruction. Its annihilation indicates a dramatic turning-point in the novel. It was obliterated as the following extract from chapter 11 illustrates:

...the conch exploded into a thousand white fragments and ceased to exist.

It is no coincidence that Piggy died at the same time as the conch. He was the one who thought rationally and who insisted on an adherence to the rules. He believed that they would only survive and be rescued if they maintained control.

Although there had been a move away from civilized order, specifically by Jack and his hunters, who called themselves savages, painted their faces and formed a tribe, their bloodlust and savagery truly came to the fore once the conch was gone. It being out of the way meant that there was nothing to hold them back. They were then able to practice their malice without any restraint, and this is exactly what they did. Jack gleefully and remorselessly celebrated the conch's destruction:

Suddenly Jack bounded out from the tribe and began screaming wildly. “See? See? That’s what you’ll get! I meant that! There isn’t a tribe for you any more! The conch is gone—”  

Once the conch was no more, nothing was sacred to Jack and his tribe. We read in chapter twelve that they purposefully set out to hunt Ralph, flush him out, and kill him. Ralph was nothing more than an object to appease their savagery - he had become much the same as the pigs that they had so gleefully hunted. It was fortunate that the naval officer found Ralph before Jack and his tribe reached him. The officer's arrival meant the return of civilization, the restoration of order and the end of savagery.