Whether you need to start over with this form or not depends upon the extent of your changes but if it is just this error then it can be resolved.
- Open the designer window for the form to trigger the YSOD.
- In the top right corner is the
Go to Code
link, click it. - You should be thrown onto the line of code that cannot find the identifier that is listed in your error.
- Fix the error
- Close the
designer.cs
file and the designer window should refresh correctly.
To fix the error, it depends upon what change you made.
If you renamed the field then fix the identifier in the designer file to match it. If you removed the field then remove all references to that field in the designer file.