I believe that one of the partition tables is corrupted or inconsistent. I strongly advise backing up any important data immediately before attempting any repair.
To delve a little deeper you can try running parted (not gparted) on the drive and see if it gives you any more information. parted tends to dump the data to the screen in a more raw format, whereas gparted tries to display it nicely and may not be able to show anything if one of the tables is corrupted.
Open a terminal windows and just type
It will complain that you are not root. That is OK, we don't want to edit or change anything anyway.
It will display some basic information including what device it is currently looking at. This should be /dev/sda. The prompt will be just
type the following to get a list of what parted thinks are the partitions on the current device:
You should get a list that looks something like the following:
Model: ATA MAXTOR STM325031 (scsi)
Disk /dev/sda: 250GB
Sector size (logical/physical): 512B/512B
Partition Table: msdos
Number Start End Size Type File system Flags
1 32.3kB 21.0GB 21.0GB primary ext4 boot
2 21.0GB 41.9GB 21.0GB primary ext4
3 41.9GB 250GB 208GB extended
5 41.9GB 44.1GB 2147MB logical linux-swap(v1)
6 44.1GB 250GB 206GB logical ext4
Paste this text table back into your question if you wish.
Given that we've now established that there appears to be a problem with your partition table, I hesitate to make any recommendation because these these usually end very badly.
You might want to have a look at this very good article on Ubuntu Forums where this exact problem is discussed. Maybe you can get your most critical data files onto a cloud backup server for recovery later. Ubuntu one is a free service up to a certain limit; just be sure that your data really is backed up and that you can access it when you need to.