I have upgraded from yeoman 1.3.4 to 1.4.0 and I cannot import mystrings.d.ts

This is due to yeoman 1.4.0 using TypeScript 2.4.2 which starts to phase out .d.ts and @Typings.

So to fix rename "MyStrings.d.ts" to IReactSearchBoxStrings,ts.  Change the contents to 

 export interface IReactSearchBoxStrings {
  PropertyPaneSearchOptions: string;
  BasicGroupName: string;
  DefaultSearchResultsPageUrlFieldLabel: string;
  EnableCustomSearchPlaceholderLabel: string;
  SearchLabel: string;
  CustomSearchPlaceholderModeOnAlert: string;
}

change

import * as strings from reactSearchBoxStrings

to 

import { IReactSearchBoxStrings} from './components/IReactSearchBoxStrings';

and finally add 

var strings: IReactSearchBoxStrings;

Then it all compiles and links.

Regards

Nigel