I have changed the fuse to "Ext. Clock; Start-up time: 6 CK + 65ms" by using the ICSP interface near the reset button. Now, I am not able to restore the fuse to "Ext. Crystal Osc. 8.0- MHz; Start-up time: 16K CK + 65ms" as the board has become unresponsive. (I was using AVRISPmkII programmer to access the mega2560 board through ICSP interface). Any suggestions please ?