In a Word Processing document, if you hold down the Return key for more than four pages, Slow Keys from Easy Access will activate itself. This can actually happen in any application that will accept the return key held down.
When Slow Keys activates, it may appear that you are no longer able to use the Keyboard. Actually, you still can, but you have to hold a key down for a few seconds before it appears.
The solution is to open the Easy Access control panel and turn Slow Keys off.
NOTE: Rather than using a bunch of return characters to move to a next page, use the Insert Break command in the Format menu to insert a page break character. Pressing the Enter key does the same thing.