Have you just installed a new plugin?
Have you just upgraded your bukkit/spigot version?
Make sure everything is compatible.
Go to the server startup log and see if there are any problems/errors initializing the NPC plugin and if you see any, google those specific problems.
Do /pl on your server. If you do not see the NPC plugin listed, it is not being recognized by the server, however, this is unlikely. If it is red, it is recognized by the server but it is not functioning correctly. This is usually due to a compatibility issue(see above). If it is listed and green, then it should be functioning properly and I have no idea what is wrong. Try doing the /npc command while keeping an eye on the server log and see if an error message appears.
Also:
VolcalicJaguarTry updating/re-installing the NPC plugin.