1989 300E; Heat and Air

I hate reposting my messages and harping over this. But I have a 1989 300E and my heat and AC worked fine one day and the next day I suddenly got no
heat nor air. THe blower motor was making no loud noises; everything was smooth. Can any of you guys direct me again as to where to look again in regards to repairing this.
-Charles, NJ
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