Visual Studio Solution Unavailable (reload doesn't work)

I faced this problem recently. The procedure below worked for my solution:

1. Right click the project which is not loading, in VS Solution Explorer
2. Click on Remove and confirm the removing process
3. Right click the Solution and select Add > Existing Project
4. Browse to the .csproj file in the project folder

If the above procedure does not work, then check the .NET targeted framework. May be Framework Incompatibility must be also one reason of the problem

If it shows error while you perform above steps then please check this

You do not have permission to access the IIS configuration file - Web app error

I was having the same problem on VS 2017 CE. I opened a .sln and the solution was empty (unavailable).

This solved my problem:

1. open the .sln file in VS
2. right click on the project name (unavailable) in the solution explorer
3. click on "install missing..." (i don't remember the exact text)

Have you tried re-adding the project to the solution from within VS? That can fix an invalid path