PDF Fields cannot be filled

I have been given a PDF by a company that is providing training with a range of fields to fill in with answers. This document is 139 pages long and is intended to be used as a training log book of sorts. I have been filling this in for a couple of months now and am about 2/3rds through the book and all of a sudden the fields have disappeared and I can no longer enter data or edit older answers. How can I turn these back on without losing all the work that I have done so far? 

I also just spent the last 2 hours copying and pasting my answers into a new copy of the document and the forms have disappeared again at about the same point in the document.