No external storage available - Storage gone While I organizing files on my Galaxy Nexus via PC (USB), Gallery app kept saying No external storage available Then I restarted the device, and whole storage was gone..

What could possible cause this? And how can i prevent this happening in the future?

Using stock 4.0.4 (IMM76I)
It looks like the phone can no longer connect to the SD card. Since this survives a reboot, it's most likely a hardware failure.

If your phone is rooted, adb shell su dmesg should contain messages that confirm this.

The “external storage” in a Galaxy Nexus is actually some non-removable flash storage. I don't think you can replace it (but I could be wrong).

It is possible that the filesystem is merely corrupted, in which case reformatting would work. You need to have a rooted device for that, and to install some additional tools from a Linux distribution.