adobe acrobat signed me out randomly, now i cant type in the PDF

i was filling out the boxes on my coursework (PDF on adobe acrobat online - on a macbook) this morning. I then left the laptop and returned 3 hours later. Adobe told me i needed to sign in again (this has never happened before?) then when i signed back in the PDF i was filling out will no longer let me type in any of the boxes? Help me pleasseeeeeee