This guide covers what it is, where to find it, how to back it up, how to encrypt it, and how to recover from corruption.
Corrosion, bitrot, and device failure are real. Use the 3-2-1 rule : three copies, two media types, one offsite.
For any non-trivial amount of bitcoin, follow the :
pywallet is an open-source Python script that can extract keys from corrupted wallets. You will need Python installed. pywallet --dumpwallet --wallet /path/to/corrupt/wallet.dat