Possible brake light switch or fuse failure - brake lights working? ..
Also possible 'syrupy' liquid has spilled into shifter cavity i.e. coffee, soda and now colder temperatures has made the problem show up .. There is an electrical solenoid down in there .. The syrupy goo has become more viscous and is keeping the solenoid from releasing the shifter ..
'Sometimes' WD-40 sprayed down into the shifter cavity will help free up the shifter interlock.
Good luck with it!
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IF all else fails: http://www.mercedesmedic.com/mercedes-transmission-shifter-stuck-park/