Why the "sdcard" partition disappeared after reboot in my Android device?

[AC Question]

After I reboot my Android device, I found that the "sdcard" partition was disappeared, the "sdcard" partition, which is a partition of the device's emmc. The question also means after reboot, the "sdcard" partition was unmount, and didn't display in the file manager.
The emmc is Samsung's, With a capacity of 16G. So grateful if anyone could solve my problems!

 

[Tim1954]

Pull the card out, give it a gentle clean and try again.
You do mean reboot and not factory reset? These are vastly different..