Depends on who you bank with, for example with Barclays when I use my debit card it will appear with 'pending transactions'.
I think I would give customer services a call, and say to them what you've said. "Why did you fail to take the monthly fee? Is there a problem on my account?". If you pay by direct debit, did it fail? Your bank can tell you if PlusNet requested it.
Also "Why is my account restricted when I have paid yourselves?". I assume it has been?
When I had a billing issue, PlusNet were very good.
So my advice is to give them a call and hopefully a CS member of staff can check the status of your account.
A member of staff may pick this up.