I don't know if this is the problem or if it would cause a problem, but you said you ran it in single player. Tekkit isn't single player, but Technic is. If you ran the world on Technic then tried to run it on Tekkit then that might be your problem.
Slefje is correct. If you have the latest Essentials then that's your problem. Tekkit uses CB 1.1-R4. So all your plugins need to work with that craftbukkit version.