There could be several causes:
The baud rate may be set incorrectly.
The transmit and receive data lines may not be connected properly, or may be reversed on one end of the connection. Check the documentation for your device to see if you need a "Straight-Through" or "Crossover/Null-Modem" cable. Many Cisco switches with a DB-9 connector on them require a "straight-through" cable. For more, see the article Crossover or "Null Modem" vs. Straight Through Serial Cable .
Flow control may not be configured correctly on either end of the connection. You need to check that the cable you're using supports the required flow control lines, and that Serial is using the correct type of flow control. Check the flow-control settings (under Terminal -> Settings…).
If the terminal is configured for "Line Buffered" send mode, no characters are sent to the remote device until the return key is pressed. If this is unexpected, change the send mode back to "Immediate".
Some devices do not echo characters back to the terminal as you type. In this case, you may need to enable "Echo Sent Characters" within Serial in order to see what you're typing.
Finally, sometimes a driver conflict can cause this to happen. Contact support for instructions on removing the manufacturer or third-party drivers that are causing this to happen.