(Sorry this response is a bit late... evidently your classmates aren't checking the Q&A frequently enough!)
This happens when you try to read from a Scanner, but there is no input to get. In particular, it happens when the scanner reaches the end of the file, and tries to consume more tokens after that.
To fix this, your getNextSentence(...) method needs to add a check for "&& input.hasNext()" to the while loop, to make sure that there is still more file to be read.
(For this particular sentence reading program, this is particularly a problem for files that don't end with punctuation...)