"UNP" reference on a credit amount?

I have a loan with Barclays and use my Barclays current account solely for paying off this loan. I have always ensured that funds are in the account in time for the monthly payment to be made. However, I neglected to do this this this month (as an error on my part, not through lack of funds). I realised on the same day that the money was due to go out that I'd not put the funds into my account so I immediately went to my local branch and transfered the correct ammount. Upon looking at my account online I see that the money was taken from my account, moving the balance to a negative (there's no overdraft), the money that I paid in is then added to the account moving it back to normal and then the loan amount that was removed has been added back with a "UNP" reference (does this mean "unpaid"?)

I'm obviously going to contact my bank regarding this but can't do so until tomorrow (Barclays telephone banking is abhorrent and I've encountered nothing but trouble from using it) and thought that the knowledgable folk here would probably know what had happened and what the likely repercussions/best course of action would be.

Thank you.

n.b. I realise that in future it would be best for me to alter the direct debit so that it's not coming from my Barclays account and this is what I shall be doing.