Transaction on my bank statement I definitely didn't make

[JollyPirate]

Don't know quite where I stand with this. In a nutshell I went to Disneyland Paris last week ( Gift from in-laws).

Now I have very little money mid-month and DH paid for virtually everything. On the second day I tried to take out ?20 from my account as I knew there would be enough there. But the ATM declined my card and came up with "insufficient funds". I was surprised but didn't question it. However I have just checked my online account to find that it is showing the transaction took place even though I know it didn't. I know it's only 20 euros but even so it's money I cannot afford to just lose without a bit of discussion.
Am I likely to get a short shrift from the bank?

Sometimes they 'set aside' funds for withdrawals or card payments which seem to go from your account, but if the transaction is cancelled it comes back, IYSWIM?

This happened to me with a train ticket machine that stopped working before the sale was completed. The money appeared to have gone out of my account but then reappeared about 5 days later.

Ask the bank if its something similar.