If the power reaching the USB device surged for any reason, the drive might fail or the data might get corrupted.
If the flash drive has low quality components, one of the components might fail and thus the drive might stop working.
Dust particles and other contaminants can prevent the connectors from connecting properly and thus the device might not work as desired.
If you force the drive in or out of the computer with high pressure, you might damage the soldier joints or weaken them; thus forcing the device to stop working.
Continuous use of the flash drive may result in errors in the memory chip that uses a NAND component. The lookup table, which is the part that helps the drive make sense of data, can contain errors and thus the drive might become unreadable.
If the drive was dropped, the solder joint might fail. In such a case, the device might not be recognized or it might show a smaller memory capacity when inserted into the computer. Dropping the flash device often can result in this problem.
High temperature can reduce the retention duration or the amount of time where the flash drive can keep the data successful. Make sure you are storing the drive in a normal temperature.
High humidity and high temperatures might result in a short circuit somewhere in the drive; thus making it faulty. See also why cellphones explode.
If the USB connector gets broken due to any reason, such as dropping the flash drive, the device might become unreadable. In such a case, the data can still be recovered if the connector is welded or replaced.
The flash drive is not designed to always be plugged in the computer. The voltage regulator part might fail as a result, and so the drive might overheat and stop working.
If you remove the drive unsafely, the data might get corrupted. In addition to that, the main file system of the flash drive might get corrupted; thus the device might be inaccessible.
If any internal component of the drive fails, the USB drive might fail. The micro-controller, the capacitor, the resistor or the oscillator might get damaged, and thus the drive might stop working. In such a case, the data can still be accessed if the faulty component is replaced.
Viruses can lead to corruption of the data on the drive, damage to the file system or even failure of the drive. Using a good virus scanner can prevent this problem from happening.