There are two parts in a payment card transaction:
- the authorisation (which usually happens at time of purchase, and shows up as a "hold" in your case),
- and then the actual payment, which usually happens overnight in batch processes.
In most cases the payment will be linked to the authorisation (hold), which is released at this point, replaced by the actual payment, so you go from having a hold on the amount to having an actual debit of the amount (not necessarily the same, in restaurants or gas stations for instance).
But in some cases the two are not linked, which can lead to both a hold and a payment (until the hold is automatically released, usually a week or so later), or, usually temporarily, neither hold nor payment (the hold has been released but the payment hasn't gone through yet). The latter case is quite unusual, but I've seen worse things happen.
The issue may be on the merchant's side or your bank's. Unless your flight is tomorrow or in the next few days, I'd wait tomorrow to see if the issue corrects itself in the next round of batches overnight.
If at that point things don't get to the correct situation (or today if your flight is very soon), you'll probably have to call the airline. They're likely to tell you that everything is fine (as their systems will tell them the payment has been processed, even if it possibly hasn't), but you can at least try to have them make a note on your file that you alerted them to the issue.
If the issue does not correct itself, at some point their systems should become aware that the payment has not really gone through (a bit like when a check bounces). What happens then is probably variable from one airline to another. Some (I hope that's "most") will try to contact you to make a new payment (the ticket status will probably change to "awaiting payment" or something similar until then). Others may just cancel the ticket outright, though I would think this to be quite rare.