Bad battery. Has just enough juice left to light the dome light briefly without any other electrical consumers on, but the moment the key is turned, there's not enough to even light up the gauge lights, never mind to spin the starter. Power door locks work because they are vacuum-operated, not electrical. Take the battery to get it tested to confirm. While you're replacing the battery, take a moment to clean up the connectors on the battery cables and the ground connection. If the battery is relatively new, you'll have the additional problem of troubleshooting the reason for it being drained. But if it's at least 5 years old, it's run out of life. Welcome to the forum.

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1988 California version 260E (W124)
Anthracite Grey/Palomino
Owned since new and still going strong and smooth
MBCA member

Past Mercedes-Benz:
1986 190E Baby Benz
1967 230 Inherited from mom when she downsized
1959 220S Introduced me to the joys of keepin' 'em goin'

There are only 10 kinds of people in the world--those who understand binary and those who don't