From Chabad.org: The details of the punishment are now revealed. The Israelites will wander in the desert for forty years. During that time, all males over the age of twenty — with the exception of Joshua and Caleb — would perish. The next generation would enter the Promised Land. The ten scouts who brought back the frightful report died immediately. When the Jews were informed of G‑d’s decision, they lamented and grieved. A group of people awoke the next day and decide to “go it alone,” and enter the Land of Israel — this despite Moses’ warning that their plan would not succeed for it was not sanctioned by G‑d. This group is cut down and massacred by the Amalekites and Canaanites.
What bothered G-d the most about the spies’ report and subsequent Israelites’ fear was the lack of trust they had in Him after all he’s done. G-d felt that they alienated Him, abandoned Him, which is why it was only appropriate that they then became unworthy of the Land of Israel. In fact, the Midrash equates this mistrust to the sin of the Golden Calf, and G-d waited until this happened before finally punishing those between 20 and 60, not allowing them into His land. If there’s any good news in this Aliya, is that after these events the nation “mourned greatly”, presumably realizing their mistaken in not trusting in G-d and/or appreciating what they had (Passuk 39). Better late than never, I guess.