Contact the owner immediately to inform them of what has happened. Depending on the lateness, the owner could ask you to pay an extra fee. Please be advised: write down the return date and time on the rental contract.
Anticipate that the owner may plan a departure several hours after your return, so be on time in order not to delay him.
Please note that you cannot go over the return date because your insurance will not be effective beyond that date. You would then be liable to pay for the repairs of any accident that might happen.