considering an ofdm symbol of x(0)x(1)x(2)x(3)x(4)....x(n-2)x(n-1)x(n), to perform cyclic prefix we prefix some samples from end of this ofdm symbol to the begining of the ofdm symbol, 
like x(n-3)x(n-2)x(n-1)x(n)x(0)x(1)x(2)x(3)........ x(n-4)x(n-3)x(n-2)x(n-1)x(n).

Instead of taking x(n-3)x(n-2)x(n-1)x(n), if we prefix with zeroes what will happen?