Keep them warm for now, but keep an eye out for momma coming back. Day old chicks can't last long before they crash out from exhaustion, so they may not be gone long, or you may be able to get her back on the nest to finish up.
Sounds like she's pretty clueless on how this is supposed to go though. Unless the one chick hatched a day or two ago, she should still be on the nest with it.
Keep their food and water close to the nest so they don't go far and see if you can get her back with them. Hopefully, it was just a quick break and the chirping pips will get her back into hatching mode.