Here are some basic TNC rules (from WL2K For Dummies)

RULE #1
Avoid Hard Resets like you would the plague.
- One hard reset in a lifetime is sufficient. Learn more about your TNC and the programs you are using so you can apply RULE #1.

RULE #2
Never turn a TNC OFF while a program has that comport open.
- This is a sure fire way for the need for breaking RULE #1. Shut down any program that is addressing the TNC before turning it OFF or starting up another program that addresses the TNC. All (good) terminal programs reset the TNC to cmd: mode when the program is shut down. Most terminal programs are expecting the TNC to be in terminal mode when the program first addresses the TNC.

RULE #3
Be sure the battery in the TNC is good.
- If it is not, non-default parameters you have been using like MYCALL, DAYTIME, MYPBBS, HBAUD, ABAUD, TBAUD, PORT (default Kantronics dual port,) TXD, etc, may not be saved and you'll have to reestablish them each time you turn it on. Otherwise, the default MYCALL will be NOCALL, PK232, etc., and the default mode may not be command mode.
- These batteries should last for five to ten years. But, that TNC you picked up at a hamfest for $35 was manufactured in 1989.
(The capacitors are that old too.)