My E320 stopped in traffic and would not restart( would not crank at alll). 15 minutes later it cranked up and my wife drove it home. 2 weeks later she drove around all day and parked the car in the garage. An hour later went to get in and the car would not crank again(does nothing no starter action). I pulled crankshaft position sensor which keeps the car from starting but still allows it to crank. I am wondering if the key could be keeping it from starting? Any and all suggestions would be helpful

 

K40 starter relay in black box at back of engine bay most likely.
CPS doesn't impact cranking as you say.
Simple & inexpensive to replace.
Known problem component.