I think the only response for the original poster is to see a Lawyer to determine the next steps. Being banned from the US can only be dealt with a lawyer.
But there are many people reading this forum who are on TN status and who would like to know more details if possible.
I have entered the US many times and the most difficult question I have heard is "what type of engineer are you"? I never know how much detail they want. I usually start very high level and they go into more detail if needed. It has always been 1 or 2 min discussion with the CPB and then I am admitted.
I have heard there are several reasons one could be banned from the US.
1) discovering prior criminal convictions (always disclose)
2) lying to a CBP officer. In the interrogation you say one thing and then later you say something else. (essentially admitting you lied originally) - never lie to CBP.
3) admitting prior drug use (even pot, which is legal at the state level in several states, but still illegal at the federal level)
Now specifically to TN status:
4) showing immigration intent. this is tough one. filing PERM, filing I-140, approved I-140, typically have not been considered immigration intent in the past. only filing i-485 has been. has this changed?
5) this CPB feels your original TN was issued incorrectly, example, the economist category has tightened the rules. ( i understand TN is the only status CBP can challenge, H1B, and L1 can only be challenged by USCIS)
6) they suspect fraud or they suspect you are not really working for your original TN sponsor. (carry your employee badge with you, carry recent paystubs with you).
My understanding if you are a law abiding person with a solid job at a solid employer, there is absolutely no reason why the CBP would deny entry to the US.
Please provide more information , so we can all learn from this experience.