C# form development visual studio on Windows uses the form editor to disappear and cannot be opened and cannot be found, the main reason and the solution

C# form development visual studio on Windows uses the form editor and disappears, cannot be opened, and cannot be found, the main reason and the solution.

The main reasons:
1) It may be that the opening method is wrong;
2) In the window program, the first class is not Form, but other classes are written;

Examples of errors are as follows:

solution:

Close visual studio, reopen the sln solution file, reopen the project.