Storage devices exist to hold information. In spite of how much you have paid for the hardware, the data itself is the most valuable asset. This is why the worst thing possible is the loss of this data.
There are many ways to lose the stored data, for example:
The drive or storage media can fail or be damaged
An accidental data deletion may occur
The data can be (erroneously) deleted intentionally
Your computer can be hit by a virus
And, if the damage to your data is irreversible, then a permanent data loss occurs.
The reasons for drive failure vary from a simple bad connection (which is easily recoverable) to damage to the media itself, which could still be recoverable, but much less frequently. If the damage occurred ...