Maxxum 7000 I have a maxxum 7000. I changed the batteries incuding thr litium but it won't turrn on.

The lithium button cell is only a memory backup for when the main power pack is off the camera. It allows the camera to retain the number of exposures, asa setting, etc. You problem may lie in the battery holder. The BH70 was prone to corrosion at the contacts and the pivot points where the part that holds the batteries to the side plate with the retaining screw. It is possible to remove the batteries from the holder, then carefully separate the two pieces by popping one end enough to allow the pivot pin to slide past the notch. Use a hard eraser to clean all of the metal points (pivot contacts and the springs and plate in the 4 cell holder.