Under the usual terms the deal simply ends — quietly, at a specific time, with nobody having to do a thing. That cuts in both directions.
A condition expires at a stated time on a stated date, and removing it means delivering something in writing before that moment. If nothing is delivered, the contract ordinarily comes to an end and the deposit is ordinarily returned to the buyer — because the condition existed for the buyer's benefit and was never satisfied.
“Ordinarily” is doing real work in that sentence. The wording of your contract governs, and if this actually happens to you it is a question for your real estate lawyer rather than for a web page.
A condition deadline is not a target. It is a trapdoor — nothing has to happen for you to lose the house.
Buyers lose houses this way. Not dramatically — the broker is waiting on one document, everyone assumes there is more room than there is, and the time passes. A seller who has been sitting on a stronger offer is under no obligation to remind anyone.
Sellers get caught too, from the other side: a buyer who has gone quiet is often a buyer who has already decided, and the seller has spent that entire period off the market. That is one of the reasons sellers read conditional offers so carefully.
An extension changes the contract, so it requires the seller's written agreement. They can refuse, and if the market has moved or another buyer is waiting they sometimes will.
The practical rule: ask the moment you know, not on the final afternoon. A request that arrives three days early with a reason attached is usually granted. The same request at 11:55 reads as a buyer who is having second thoughts, and gets treated like one.