You did not mention in your post whether there was any trickle of air coming from the vents, and if there was some air, was it cool or just ambient?
If the system was working fine yesterday, and the blower failed to come on, first check the fuse on the blower circuit. It is located in the fuse box mounted near the firewall (not sure about a 400E, but on an E320, the fuse/relay box is on the firewall right at the master brake cylinder).
If the fuse is ok, and there is a trickle of cold air coming from the vents (which you shouold notice when the car is moving), then in all likelyhood, you have burned up the blower motor. I have had three Benzes in the last seven years, and everyone of them has seen the HVAC blower motor fail at anything from 100k miles up to 185k miles on my 300E.
Mode of failure on these blowers is that the motor carbon brushes just wear out with time.
Cost to replace the blower motor is around $400 including the new motor.
Hope this helps.