Having funds deducted out of your account that you never received is an obvious problem that nobody wants to be faced with. There are several things that could have happened here, but ultimately the bank will have the final decision on what is being done to rectify the situation.
All ATM machines are monitored with video. It will be easy enough to tell if the individual actually received money via the machine through the video system, as long as the tape has not been cleared. There are also safety mechanisms within the machine that are able to note any challenges the machine faces when it disburses money.
It is important to contact the bank ASAP and file a complaint. If when they settle the machine they find the balance to be off, your complaint should be immediately rectified and the funds returned to the account. If any fees were incurred because of this mistaken transaction, they should also be reimbursed.
One thing we would like to make note of here is if the funds were somehow caught up in the machine. If this were the case, the machine would balance, but the person who made the next withdrawal may have benefitted from the mishap.
It is something that would be very tough to prove, but hopefully the honesty of the next user would prevail.