Did you witness any problems with her straining to have them?
It is very unusual to lose a whole litter, but it is possible that one got stuck, and blocked the way out for the rest.
A piglets umbilical cord is only 50cm long, and the track to get out about 1.5m, so the cord breaks inside the sow. The piglet has about 5 minutes to get out before it dies of suffocation, so if one piglet blocked the exit, then it is possible that the others backed up and all suffocated. Most unusual, but if there are no other signs of illness, then that's the explanation I'd go for.
Suffocation is by far the most common cause of a piglet being born dead, but otherwise being fine and fully formed. If they don't fully form then they look mumified, and if they die other than immediately before birth they tend to be re-absorbed, mummified or very odd looking.
The sow has two uterine horns and a central way out - like an "M" with the middle "v" bit of the M being the exit. Sometimes they can come up one side (eg from the bottom left of the M, and miss the central exit, going down the other (right hand) leg of the M. They can then take too long to exit and suffocate. Also the last from one side of the horn has a long way to travel, and it is not untypical for half the litter to be born fully alive, then one dead, then the other half all live, - the last on one side didn't make it out in time. Ditto why you can often get a dead one with the afterbirth.
I'd certainly try her again