Only Achan took. Why is the sin attributed to Yisrael?
Malbim: All of Yisrael are tied like one body. Each person relates to the Klal like one limb of the body to the body. Just like through illness or loss of one limb causes illness or blemish in the entire body, so Achan's taking from the Cherem is considered as if the entire nation transgressed.
Malbim: There is a difference between a punishment that comes actively, from one that comes from removing Hashgachah. In the former, others are not punished due to the sinner. Achan alone was stoned. However, removal of Hashgachah affects the entire Klal. When there are sinners among them, Hashem hides His face from them
What was the consequence of "va'Yichar Af Hashem bi'Vnei Yisrael"?
Malbim citing Mahari: He removed His Hashgachah from them.