The maximum time for a hen to be off the eggs is 20 mins so they say, although I have to admitt to shutting the door on one of mine for half an hour :shock: they were ok.
It's funny you should say she's a buff Orp I had one do very similar this year with duck eggs 4 days before they were due, it was asif she'd heard them and didn't want to know.
If we're talking more than an hour here well I think they might be gone but at this late stage the babies will be moving therefore generating some heat for themselfs
Get her back on them and keep your fingers crossed.
She might reject once hatched, mine did but with some tin foil, empty container such as a fish tank and a light bulb I can talk you through making a brooder as and when needed
Hope things go ok , My fingers are crossed for you