Lost external drive

I recently upgraded to 16.04 and as a result lost access to my main external drive. error msg as follows:
Error mounting /dev/sdc1 at /media/jucick/Ubuntu 16.04.1 LTS amd64: Command-line `mount -t "iso9660" -o "uhelper=udisks2,nodev,nosuid,uid=1000,gid=1000,io charset=utf8,mode=0400,dmode=0500" "/dev/sdc1" "/media/jucick/Ubuntu 16.04.1 LTS amd64"' exited with non-zero exit status 32: mount: /dev/sdc1 is write-protected, mounting read-only
mount: wrong fs type, bad option, bad superblock on /dev/sdc1,
missing codepage or helper program, or other error

In some cases useful info is found in syslog - try
dmesg | tail or so.
please help this is a 3 tera drive which is near full of my must important data.
Tks