I had this situation. Try to do the following.
1. Create a new similar project
2. Copy the content of the *.csproj (or vbproj, etc.)
3. Remove all the "Compile Include" references.
Basically this:
<Compile Include="Form1.cs">
<SubType>Form</SubType>
</Compile>
<Compile Include="Form1.Designer.cs">
<DependentUpon>Form1.cs</DependentUpon>
</Compile>
4. Launch your project, it should starts with none of your forms, class...
5. Add all your existing items in your project...
In the extreme case, if this advice does not help, then try to use the service of online repair Microsoft Project. Maybe it will help.
https://onlinefilerepair.com/en/project-repair-online.html Hope that helps!)