Probably activated the anti-theft system, if you replaced ecu it may need to be programmed to work with current key. Either find a qualified locksmith or take it to the dealer.
The eis,key,ecu,shifter all from a working car
The computer modules on your car all need to be programmed to work together for the anti-theft. You can't just swap out one of them with reprogramming it, the ECU especially.
Looking for a Used S-Class in your area?
CarGurus has 124 nationwide S-Class listings starting at $5,999.
Search Mercedes-Benz S-Class Questions
Mercedes-Benz S-Class Experts
Related Models For Sale