Mercedes-Benz SLK-Class 2008 Owner's Manual page 211

Practical advice
Troubleshooting
Problem
The key cannot be turned in the
ignition lock.
208
Possible cause/consequence
The key has been in position 0 for
a considerable time.
The on-board voltage is too low.
Suggested solutions
Remove the key and reinsert it into the ignition lock.
X
Check the starter battery and recharge it if necessary
X
( page 232).
Y
Start the engine.
X
Switch off non-essential consumers, e.g. seat heating* or interior
X
lighting, and try to turn the key again.
If this does not work:
Check the starter battery and recharge it if necessary
X
( page 232).
Y
or
Jump-start the vehicle ( page 233).
X
Y
or
Consult a qualified specialist workshop.
X
* optional