Do not write anything onto the partition which contains your important files.
First, confirm that the partitiion you think to be the partition which contains your important files is really the partition which contains your important files.
or palimpsest (gnome disk utility) or gparted may help you to confirm.
sudo parted -l
If it is really the partition which contains your important files, then confirm that you have enough access permission to the directories and the files in the partition.
Replace the above words between "<" and ">" to proper word.
sudo ls -l <mount point of the partition>
Do not type "<" and ">".
Read manuals to understand the output of "ls" by
You can check the mount point of the partition by
If the partition is not mounted, you can mount it onto /mnt (mount point of the partition) by
I assume you use mint. I have a poor memory, but I remember that some linux distributions like sabayon mount some partitions onto a directory (such as windows) in /mnt. If you use such distribution, mounting onto /mnt may mess up your system.
sudo mount -t auto <device file name of the partition> /mnt -o rw
Device file name is like /dev/sda5.
If you do not have enough access permission, then you can change the owner of them by
You can also change the permission of them by
sudo chown <your user name (account)> -R <mount point of the partition>
You can read manuals by
sudo chmod <permission combination> -R <mount point of the partition>
If the partition which contains/contained your important files does not contain any of your important files, you might have formatted it.
Even so, you may be able to recover files by "photorec" or "testdisk"
You can install photorec, which is involved in "testdisk" package, and testdisk by
sudo apt-get update
sudo apt-get install testdisk