If you have an interface named (as an example) "wlan16" and you attach airmon-ng to it, then exit ungracefully (e.g., via ctrl-c), you probably notice that you now have an interface named "wlan16mon". The repair is quite easy.
1) Use iwconfig to check that the interace is still in monitor mode.
iwconfig wlan16monIf it isn't in monitor mode (e.g., you've been messing with it and changed the mode), return it to monitor mode via:
ifconfig wlan16mon down iwconfig wlan16mon mode monitor
2) Use the following airmon-ng to stop the interface and return the name to normal.
airmon-ng stop wlan16mon
3) Check the interface