I had the same problem a couple of days ago.
So I cleaned the dead slot with an eraser, and than removed the erase dust with a regular brush and after I used a mechanic pencil to put grafit on the slot so it would be more conductive.
My Macbook finaly turned on with the 2 slots working.
I still need to wait for a few days to be sure that the problem will not come back, but than I just have to try the same procedure.