I booked a hotel room through booking.com with FREE CANCELLATION policy. Once booked, a small amount of money was pledged from my debit card balance (for security reasons, for the hotel to confirm if my debit card was valid), but it was not recorded as a valid transaction. Booking.com mentioned this action indeed. I cancelled the booking from this hotel, booking.com confirmed that I won't be charged with any cancellation fees, but I still want to know about the amount that was pledged from my account.
Your mention of pledged and not recorded as a valid transaction makes me think you refer to an authorization hold - which is entirely routine in such circumstances.
As mentioned by @CptEric this is to make sure you, well, got money in there. The amount held is variable (the Wikipedia article mentions usually $1 but sometimes up to $100 in connection with purchases of fuel) and 55 euros for hotel accommodation seems perfectly reasonable to me - it may be enough for at least one night's accommodation and hence give time for the hotelier to discover that the prospective guest has not turned up so the reserved room can be freed up for others without the hotelier missing payment for the first night reserved because someone who booked did not turn up.
It is normal for hotel reservation cancellation fees to be the cost of one night's stay, though occasionally it can be the cost of the entire period booked. It is very unlikely that booking.com is the only outlet for renting out rooms for any hotel. At the very least most hotels will accept walk-in guests, if they can accommodate them. So the hotel you chose may have applied their 'standard' minimum charge of one night, whether or not the guest arrives.
However, booking.com reaches a far larger audience that any single hotel can afford to reach out to on its own and because the potential for customers is so great many hotels are prepared to take the risk of cancellation themselves. That is, they may accept that occasionally they receive no income from a room which they could have let out had they had enough notice that the reservation was not required because booking.com is sending them so many more guest-nights than the hotel could have achieved without booking.com .
But free cancellation is not unusual on booking.com so any hotel that does not offer that is likely to be less attractive than others that do, so a hotel's 'standard' cancellation charge may have to be waived if such volume is to be achieved by it. booking.com is a search engine for good value accommodation and most of its users will be planning ahead. Users will be aware that in the time between making a reservation and requiring the room circumstances may change. For example, I reserved accommodation in Lagos before I discovered that my visa application was declined without leaving me enough time to reapply. I notified the hotelier immediately and hope it was able to find others to take the space instead but even were it unable to do so the gamble for the hotel is reasonable. I had every intention of paying for four weeks of accommodation but was thwarted by The Nigeria High Commission's incompetence and the hotel missed out on a 'standard' cancellation fee of the cost of one night's accommodation. I am very sorry for the hotel but had they not offered free cancellation I would not have booked with them anyway.
Note that your hotel does not have your 55 euros, it is still in your bank even if you cannot at present access it. Normally, given a matter of days at most you will regain access to the 55 euros without your having to do anything. It is possible that someone makes a mistake and you are caught in something that is not routine but that is extremely unlikely.
It's typically called an Authorization Hold and deducts that amount from either your Available Credit for a Credit Card or Available Funds for a Debit card.
Basically, the money was never taken from you, it was just 'reserved' to cover the potential future debit.
Authorization Holds either expire or are removed when the corresponding Charge/Debit is posted.
This probably has nothing to do with any Cancellation Fee. If you read the specific terms, it's probably the first night guarantee in case you are a no-show.
How long has it been? I've seen instances of Authorization Holds lasting well beyond my expectation but not long enough to do anything about.