It means that Ubuntu cannot find /mnt/sdc1 to mount it. If you have specifically specified "/mnt/sdc1" in your /etc/fstab file, change it to use UUID's instead as the drive has probably changed designations to something else.
For now, press "S" to skip mounting and allow the system to boot, then get into a terminal and type in blkid to get the UUID's of each drive on your system.
Identify the drive you originally setup as /mnt/sdc1 and copy down the UUID which will look something like this in the output:
/dev/sdb1: LABEL="My_C_Drive" **UUID="7378084A69DC3DF8"** TYPE="ntfs" (A Windows formatted drive)
/dev/sdd1: **UUID="5fc6e3a5-a060-42f6-9368-9fded7e0d048"** TYPE="ext4" (A Linux formatted drive)
Then in /etc/fstab, place that UUID down instead of the device name so the line looks something like:
**UUID=5fc6e3a5-a060-42f6-9368-9fded7e0d048** /home ext4 noatime 0 2
In the above example, we've setup device /dev/sdd1 which is EXT4 formatted to be mounted as "/home".
Save, reboot and test.