Can't modify text in fillable fields

Someone sent me a PDF that I was filling out. Everything was working great until (I think) I signed the document. Then I saved the doc and reopened it and I could no longer modify any of the text. The editable fields used to be blue and now they are white. It is a very long form (107 pages) therefore I don't want to re-enter everything. Assuming that signing the doc made it uneditable afterwards, how do I remove the signature? In general - how can I make the doc editable again? 

Thanks in advance!

Mike