why is betty laying unconcious?

We're not exactly sure why Betty's unconscious or even if she really is unconscious in the first place. At the very least, she must've been absolutely terrified to be caught by her father dancing "like a heathen" in the forest at midnight along with Tituba and Abigail. Superstition is rife in Salem, and anyone doing something that looks vaguely indecent is liable to condemnation for being in league with the devil.
It's not surprising, then, that Parris suspects that the girls were engaged in witchcraft. Neither would it be at all surprising if Betty chose to feign illness to avoid the taint of suspicion. In any case, she certainly snaps out of her "trance" soon enough when the other girls start arguing among themselves, expressing fear that they're going to be labeled as witches.
Mr. and Mrs. Putnam are certainly convinced that Betty's been put into a diabolical trance, just like their daughter Ruth. However, Betty's incredibly lucky in the sense that her father wants to avoid any kind of scandal involving his family, so he's reluctant to publicize his daughter's "possession." At the same time, if it can be shown that Betty is the innocent victim of a witch's evil spell, then he can use it to good effect, consolidating his shaky position within the community.
From the very outset, then, we can see just how useful a witch hunt will be to at least four different people: Parris, as we've just seen, sees it as an opportunity to strengthen his authority; the Putnams realize that they can grow rich on property confiscated from those suspected of involvement in witchcraft; and Betty, for her part, can now blame her vibrant midnight dancing on a wicked magic spell cast upon her by dark, malevolent forces.