If natural resources are not managed properly and used in sustainable development, a country will generally end up with no resources and with not wealth to make up for the loss of the resources.
If a country's resources are not managed wisely, they will typically be used up quickly. The country's leaders will want to get financial benefits from the resources as quickly as they can. They will not be patient in exploiting the resources.
In addition, the money gained from the resources will not be used wisely. Leaders might use the money simply to enrich themselves. Alternatively, they might invest it in ways that are not wise. Either way, the money will be used up and there will be little or nothing left to show for it. An extreme example of this process is that of Nauru.
Thus, a country that manages its natural resources very badly will generally end up with the resources gone and its inhabitants no better off than they were before the resources were exploited.