Check for Running Processes:

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Antivirus Interference: Overly aggressive antivirus or security software might interfere with Bitcoin Core's access to its data files, quarantine them, or prevent proper writes. Some antivirus programs have been known to flag legitimate blockchain data as containing viruses, even though this data cannot infect your computer, it breaks the database.
Low Disk Space: While less direct, extremely low disk space can sometimes prevent proper file writing or indexing, leading to data corruption.
Software Bugs: Rarely, a specific bug in a Bitcoin Core version or an interaction with your operating system could trigger a database issue, though these are typically patched quickly.
Troubleshooting Steps (from least to most drastic):
Before you do anything, ensure you have a backup of your wallet.dat file! This file is paramount as it contains your private keys and controls your funds. It's usually located in your pharmaceutical email list Bitcoin data directory. If you lose this file and don't have a backup, your Bitcoins will be irrecoverable.


Ensure Bitcoin Core is completely shut down. Sometimes, processes might linger in the background.
Windows: Open Task Manager (Ctrl+Shift+Esc), go to the "Processes" tab, look for bitcoin-qt.exe or bitcoind.exe, select it, and click "End Task."
macOS: Open Activity Monitor, search for "Bitcoin Core" or "bitcoind," select it, and click the "X" button to quit the process.

In summary, if you hear or see "Bitcoin blockchain down," it's almost always a misinterpretation of a problem with a centralized service or a period of high network congestion, not a failure of the decentralized Bitcoin network itself.