My 1996 Z71 only blows air out of the floor vents and AC and C buttens dont light up Y is this

the ac does not have enough coolant to activate the system. if all the ac refrigerant has leak out the computer will not let the ac system turn on to prevent damage to the ac compressor or other ac components.

Jimmy O is probably right. You need to have someone check the AC system and see if there is a leak in a line, etc. If not, have it refilled with refrigerant.