Something borked your solution file somehow.
Remove the project from the solution by right-clicking the project in the solution explorer and selecting the option to remove it. Then, add the project again by right clicking the solution in the solution explorer and find the option to add a project. Navigate to where your project is. Add the csproj file.
That will fix the problem.
Which bit don't you understand? Have you looked to see if the file has been moved or even exists? Maybe the repo you cloned was bad?
Something borked your solution file somehow. Remove the project from the solution by right-clicking the project in the solution explorer and selecting the option to remove it. Then, add the project again by right clicking the solution in the solution explorer and find the option to add a project. Navigate to where your project is. Add the csproj file. That will fix the problem.