This usually happens because the code you have added is referencing non-secure content (non-https URLs).
Two common examples are:
<iframe src="http://www.google.com/maps/embed http://www.google.com/maps/embed .. .. </iframe><script src="http://www.myscripts.js"></script>
You can see that in these examples, the URLs reference an HTTP, not an HTTPS.
To fix this, you can change the URL to include https.
<iframe src="https://www.google.com/maps/embed… https://www.google.com/maps/embed… .</iframe>
If the widget still does not display after making the changes, then this means that the widget creator does not host its widget securely. You will need to find a similar widget whose content is hosted through https.
Please note: any third party content you add to your site should be securely hosted, regardless of whether or not SSL is enabled on your domain. This will ensure that your content displays in all browsers and your visitors will not receive any security warnings. Browsers are going to become increasingly strict in this regard, so the best approach is to ensure that all content on your site is secure (https).