This tutorial will explain how to re-sync the blockchain from scratch.
1. Make sure you have the latest version of the wallet software. If in doubt, you can get it here. See this guide for detailed instructions on how to update.
Make sure you close your wallet software before proceeding.
2. Go to your wallet's data directory. This is where the blockchain and ‘wallet.dat’ are stored.
Windows: The default path is ‘C:\Users\your_windows_username\AppData\Roaming\xtrabytes’
Mac: The default path is $HOME/Library/Application Support/xtrabytes/ OR /Users/username/Library/Application Support/xtrabytes/
3. Now select all the files and folders, except ‘wallet.dat’, and delete them. It’s very important that you leave ‘wallet.dat’ in the directory as it contains your coins.
4. Restart your wallet software. The synchronization process will start from scratch.