amazing... all of these "mechanics" replying, but NO MENTION of the possibility of a BAD RATCHET in the freewheel or freehub....

if everything else looks good/serviceable, then it was probably the ratchet in the freehub or freewheel.... YES, they FAIL. Excessive slop in their bearings is a good way to inspect them... and i've found one that had managed to back it's retaining bolt out, too! most of the walmart freehubs are not removable, and the entire hub will need to be replaced... suggest upgrading to a better hub, in that case....

i even replaced an Ultegra hub recently, that would not engage every so often... the skipping of Walmart hubs is fairly common, so if it's a low end bike, that is almost always the cause of "chain skip", if everything else is ok.

ALSO.... i took in a couple NEW Walmart/Bell chains from a scrapper last winter... they were "WORN OUT", right from the package!... i tossed them back into his next load of scrap... but bent them up so no one else would have to deal with them...

Bad ratchet springs, severe wear after RUSTING, or old (stiff) grease, are the usual causes of ratchet failure.