Hi, brahmam,
The two words can sometimes be used interchangeably, but not always. "Cause" is a little more technical/physical.
The cause of the flood was very heavy rains in the Spring.
The reason for the flood was very heavy rains in the Spring. - Here their meanings are essentially the same.
"Reason" is usually more mental than physical, though.
The reason I called you is that I need to borrow some money. (You can �t use "cause" here)
I hope these examples help you a little.
Bruce