Adobe Digital Editions authorisation error on tablet

I have been reading library books on my tablet for months.  I have now tried downloading book to my pc and transferring to tablet.  Tablet comes up not authorised.  I put in my ID & password and it says authrorised but when I try to open a book it says not authorised. This now happens when I try to download direct to tablet.   I have tried ctrl/shift D  and ctrl/Shift u without success.  Any ideas?