I have got a RB433 AH, i was trying to upgrade it from version 4 to version 5. I copied the file in the routerboard and restarted the board so that it updates the version. But during the upgrade power was plugged off by accident and now i can not see the routerboard in Winbox. I can not access it at all. Any suggestions????

Use a serial cable to set it to boot from Ethernet and then use NetInstall to reinstall the firmware. The wiki has details on both.
http://wiki.mikrotik.com/wiki/Manual:Netinstall
http://wiki.mikrotik.com/wiki/Manual:First_time_startup