worksheet

Hello! Joined this site yesterday and made my first worksheet, but today the worksheet isn't being showed properly. That pdf file that I used for making the worksheet kind of "disappeared", there is only a symbol of picture, but all interactive options do exist on the worksheet but without a background. What can be the reason for this? And what can I do not to face this in future? Thank you beforehand. I hope I explained it quite understandable. Looking forward to your answer.