Speaking generically, not so much of Mercedes, try pressing the brake
pedal and making sure the vehicle is in park.

First, try moving the steering wheel to release any pressure on the
locking mechanism.  This is for any car, not just MB.  If there is
tension on the wheel from the locking plate, then the innards can't
retract, therefore, you can't turn the key and start the car.

Try another key,  sometimes they get worn and get grumpy.  I keep a
pristine original key in a stash at home and drive with a duplicate,
then if it starts to wear, and they do over time, I have a nice crisp
original to have another copy made with.

If this still doesn't work, or even if it does, but the ignition is
still being snarky, get thee to the dealer and order yourself a new
ignition cylinder pronto.  They are easy to replace IF you can get the
key to turn.  Second cousin to impossible without major surgery if it
it stuck.  They do wear out.  I replaced the one in my 79 240 D about
4-5 years ago.  Luckily I caught mine when I could still get it to turn
with some patience.  Took about 1/2 hour and a paperclip to swap out
the lock cylinder, no problems since.  Now for the driver's door
lock....

Snowfire

If you pull on the steering wheel to one side does it move then? Hoe
bout to the other side?

If not your lock cylinder has failed and I'm afraid it's a bit of
a pig to replace when it has failed - not so bad when they're merely
failing though.

Happened to me this morning. On the busiest street in Manhattan at
morning rush hour. It had been getting balky for awhile but today it
quit altogether. Nothing would budge it. Had to have the car towed to a
Mercedes repair shop. Hope to have it back today.