C# – No connection string named ‘MyEntities’ could be found in the application config file

This means that the application is unable to load the EDMX. There are several things which can cause this.

• You might have changed the MetadataArtifactProcessing property of the model to Copy to Output Directory.
• The connection string could be wrong. I know you say you haven't changed it, but if you have changed other things (say, the name of an assembly), it could still be wrong.
• You might be using a post-compile task to embed the EDMX in the assembly, which is no longer working for some reason.

In short, there is not really enough detail in your question to give an accurate answer, but hopefully these ideas should get you on the right track.

Update: I've written a blog post with more complete steps for troubleshooting.

For a sample app.config file like below:

<?xml version="1.0" encoding="utf-8" ?>
<configuration>
  <appSettings>
    <add key="countoffiles" value="7" />
    <add key="logfilelocation" value="abc.txt" />
  </appSettings>
</configuration>

You read the above application settings using the code shown below:

using System.Configuration;

You may also need to also add a reference to System.Configuration in your project if there isn't one already. You can then access the values like so:

string configvalue1 = ConfigurationManager.AppSettings["countoffiles"];
string configvalue2 = ConfigurationManager.AppSettings["logfilelocation"];