Ioulia,
As Vaclav said, it's most likely that your form is not displaying because the browser is protecting the user from submitting it when there is insecure elements on the page.
That is to say, some elements (e.g. javascript files, photos, etc.) are being pulled into the page with http links rather than https. For example: http://api.google.com/jquery.js vs https://api.google.com/jquery.js.
To figure out which elements need correcting:
1. Open the offending page in Chrome
2. "View Source" or type Control-U
3. Type Control-F and enter "http:" (without quotes)
4. Type Control-G until you've cycled through all of them
To fix:
1. If the rabble-rousing elements are coming from the same domain that the page is on (e.g. http://example.com/pic.jpg on page https://example.com/form/) you can simply delete the http://example.com, leaving /pic.jpg, and it'll be fetched with https.
2.You can also change all instances of http:// to https://
Mark this thread as solved if this fixed your issue.
Cheers,
Alan
.png)