Sherem asked for a sign from the Holy Ghost that Jesus would come 500 or so years later. God gave him a sign. What was it?

God killed him.

He [Sherem] said unto me: Show me [Jacob] a sign by this power of the Holy Ghost, in the which ye know so much.

I said unto him: if God shall smite thee, let that be a sign unto thee.

When he had said these words he could say no more, and he gave up the ghost.

Now, this thing was pleasing unto me, Jacob ... for he had heard my cry and answered my prayer. Jacob 7:14-22

Here is the LDS Church's official excuse for Sherem's death