First, try shutting down (powering off) your computer and resetting the iPod
Start up the computer, and connect the iPod to a direct USB port on the computer, if you have been using a USB hub. If there are other USB devices (other than standard keyboard/mouse) connected, you may want to disconnect them, as a test.
If that does not help, or you already did that, it is possible that the iPod's hard drive has become faulty. It is also possible that the hard drive is OK, but there is data corruption that is making the iPod's storage unreliable.
If the problem is not resolved, you should probably do a Restore on the iPod in iTunes. The Restore button is on the iPod's Summary screen in iTunes. This will erase the iPod, reinstall its software, and set it to default settings. If the cause is data corruption, the Restore (erasing the storage) should fix the problem. However, if the cause is a hardware fault on the hard drive, it will not help; the Restore may not complete successfully, which will leave your iPod inoperable.