There are many reasons that this might happen. Sometimes it's due to effects, sometimes it's a shape layer, and sometimes it's something else entirely.
Is there a transparency layer on top of everything that may have been turned opaque (not transparent) when processing your proof?
You should remove, not hide any unnecessary layers in your project before outputting to PDF and uploading. This should help.
Also, we recommend viewing the proof using Google Chrome. In most cases, this will resolve the issue.
What is the file size? If it is way too large for the PDF, you might see a blank proof.
Rasterizing the text or image might help, however, if you submit a flattened image and there are small "tweaks" needed to improve the image, we'd be unable to assist you with that.
If you wish to move forward with your project, we would most likely be able to use the PDF files that you sent us, as long as the template guides were removed from the image before you uploaded them. (We will not go to print if there is a bleed, safety or resolution issue present without your consent!)