In my case, this issue was caused by a simple Javascript error, BUT because **npm start** built and ran with no errors or warnings, *I didn't know there was an error*. So I was stuck for hours.
I finally found the Javascript error **in the browser's console**, so check there.
(You can view the javascript console in your browser of choice, usually by right clicking on the page and choose **Inspect**).

In my case, this issue was caused by a simple Javascript error, BUT because **npm start** built and ran with no errors or warnings, *I didn't know there was an error*. So I was stuck for hours.
I finally found the Javascript error **in the browser's console**, so check there.
(You can view the javascript console in your browser of choice, usually by right clicking on the page and choose **Inspect**).