Effect of cyclic prefix and zero-padding in OFDM
The main idea of using Cyclic prefix in OFDM systems is because of 2 reasons, 

1) To eliminate ISI from previous symbol. 

2) To model linear convolution as circular convolution 

If an OFDM symbol is Zero padded instead of cyclic prefix, ISI due to adjacent symbol still gets eliminated, but it cannot be modeled using  circular convolution. Circular convolution converts frequency selective multipath channel into flat fading channel. Circular convolution simplifies channel estimation and equalization which can be done easily in Frequency domain.