Could not load the Umbraco.Core.Configuration

Questions
1

I am using Umbraco version 6.2.1. Everything was working fine till yesterday and today when I opened the project I am getting the following exception

Could not load the Umbraco.Core.Configuration.UmbracoSettings.IUmbracoSettingsSection from config file, ensure the web.config and umbracoSettings.config files are formatted correctly

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How can I fix this?

This is a companion discussion topic for the original entry at https://our.umbraco.com/forum/67025-could-not-load-the-umbracocoreconfiguration