A search will reveal all kinds of threads (this type of problem also occurs not only on E class, but S's as well).
First thing you can look at is the fan resistor (behind drivers headlight, under a perforated metal cover - often, the upper wire is corroded/burnt off.
Note that a non working resistor, only causes the low speed fan not to work - you didn't say wether both speeds were inoperative. The high speed fans come on if the engine temp warrants it. There is also a strip fuse that can be cracked/melted. I think on the 400E's it sits on the drivers fenderwell under a black plastic cover. Depending on what month/year yours was made, it could also be next to the brake booster.
Might want to include the year of your 124 next time - at least list it in your profile - this helps A LOT.

Good luck!

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It is a truism that almost any sect, cult, or religion will legislate its creed into law if it acquires the political power to do so. Robert A. Heinlein


09 Jetta TDI
1985 300D