if I have I've enchanted a land card w/ "Market Festival" & my opponent plays "Desecration Plague" that states "Destroy target enchantment or land" and my opponent declares he's going to destroy my land card that's enchanted does that mean my enchant card is destroyed as well? If not, can I enchant another land card?
Yes the enchantment will be destroyed, the permanent is was enchanting is no longer on the battlefield so as a state based action it gets destroyed.
{Market Festival}
{Desecration Plague}
Dat 2 for 1. Such value!
As a very small nitpick, it's not 'destroyed'. Auras (most of the time) simply 'fall off' (are put in to their owner's graveyard) when the permanent they're attached to is no longer on the battlefield. If they were 'destroyed', that would imply that making them indestructible would cause them to hang around, which wouldn't make any sense.