nisha101, talk with good immigration lawer. most of them won't charge you for one or two phone consulting. where are you in states? send me your mail id if you need lawer info or try to find local nepali lawer.

"your parents visa stamp in their passport is now cancelled pursuant to 222(g). They will need to apply for a new visa if they want to come to the US in the future and will have to explain their period of overstay and provide more evidence of their bonafide non-immigrant intent, assuming they will apply for a tourist visa."

but there are other people who argue that their parents came back without any problem even when their visa was denied.

http://murthyforum.atinfopop.com/4/OpenTopic?a=tpc&s=1024039761&f=7484034071&m=3511078171

also try searching more forum.