then after the router was reloaded, hyper terminal appeared to be blank
now what can I do to configure the register number back to 0x2102?
The config register set to 0x21012 will boot from flash, but will set the baud rate to 1200. So if you set your hyperterminal to 1200 baud you will be able to get in and change the config register back to 0x2102. To change the config register back you will need to get into global configuration by typing config t and then once in global configuration mode type config-register 0x2102. Now when you look at a show version you should see that next time you reload the config register will change to 0x2102. If you have trouble getting into the router here is a link that will explain changing the jumper to set the baud rate on the console back to 9600.
I hope this helps you get into the router.