The Torah tells us that "the people saw" that Moshe was delayed. The words: "the people 'saw,' refer to their mental eye, of course. But there is yet another understanding. Chazal (Shabbos 89a) say that the Satan came and showed them the image of darkness and the picture of Moshe Rabbeinu lying on a bier, dead. This is why the Torah used the expression וירא העם - that there were circumstances which justified the people thinking Moshe had indeed died.

      This miscalculation of the Jewish people was what led to a momentary period of confusion and depression. Explains the Mirrer Rosh Yeshivah, R’ Chaim Shmuelevitz zt”l (Sichos Mussar), since Geulas Mitzrayim, the Nation had been under Moshe Rabbeinu’s constant and continuous leadership; his guidance and direct communication with Hashem gave them stability and certainty. The moment they feared that Moshe wasn’t returning, they panicked, leading them to build the Golden Calf. This teaches us that we must be particularly alert during moments of confusion and depression in our lives, since we can be vulnerable to mistakes and errors in judgment.”