Underscore (_) is not working in field name at sharepoint

Hello,

we are working with sharepoint now we have a error with our application , it’s a windows app in this app we have some field with underscore ( _ ) as like (First_Name)

 A few days ago we are getting the whole values with underscore ( _ ) but now we cannot access these fields with underscore ( _ ) in field name while we can get field data without underscore ( _ ) like (FirstName).

thanks