What a sad verse. The story behind this is that Moses sent spies into the promised land to get a report of the lay of the land and its residents. Two of the spies: Joshua and Caleb were pumped about it. They said it was ready to be taken. The rest of them though said the residents were giants and there was no way God's children could win. They would be defeated.
The Israelites listened to the ten spies instead of the two and they wailed and cried and complained.
God heard. God was mad. God told them through Moses they would die in the wilderness and their children would have to live 40 years as shepherds in the wilderness before they entered the promised land. But they would enter.
The next day the verse quoted above happened. Moses told them it was too late. They went anyway and were beaten down by the enemy.
When we sin especially with unbelief and fear, we miss out and we can't get that God opportunity back. God can and will restore us when we turn back to Him but that opportunity is gone. We will never know what we might have seen or had or done.
Grumbling is in the present too. When we grumble we are looking at ourselves rather than God or we are looking at others and complaining about them.
In Numbers 11 the people complain about God's provision.
In Numbers 12, Aaron and Miriam complain about Moses, God's servant.
In Numbers 13, the people chosen to do a God-task complain about the obstacles to the opportunity -- promote fear instead of faith in the people.
And in Numbers 14, the people refuse to believe God can overcome and cry because they are scared.
All of these kinds of complaints still happen among God's people. "There is nothing new under the sun."
Please don't be a grumbler.