Assuming you are connecting over a console cable, hit the break key combination when as the router starts up. The "break" key combination will vary depending on what you are using for your terminal emulation. In Hyperterminal, I believe the combination is the Ctrl-Pause/Break button on the keyboard. If you don't know what the break command sequence is, you can try the following steps (although I've never had to test it)
1. Connect to the router with these terminal settings (over console cable):
* 1200 baud rate
* No parity
* 8 data bits
* 1 stop bit
* No flow control
note: You will not see any output on your screen.
2. Power cycle (switch off and then on) the router and press the SPACEBAR for 10-15 seconds in order to generate a signal similar to the break sequence.
3. Disconnect your terminal, and reconnect with a 9600 baud rate.
I'm assuming you are attempting a password recovery on the router.. in which case, please review Cisco's docs on the procedure.