Dual circuit receptacle problem

One double plug in Kitchen has only 1 part working. Opened it up and found that there is 240v between Black and White wires 0v between Red and White wires 40v between Red and Black Does this mean Red wire is short somewhere i.e. with no current. More so because one area next to Kitchen i.e. bathroom and porch is out of power also. How do I find the source of this short. How do I go to the junction box. Basement is drop slab ceiling hence reaching the wires etc. is only a matter of moving the slabs and looking at the wood frames and wiring.