How to Re-sync the Blockchain (Windows) This tutorial will explain how to re-sync the blockchain from scratch. Follow this guide if you are experiencing problems synchronizing the blockchain, or you are getting error messages when starting your wallet software. Some times a bug can occur that either prevents your wallet from synchronizing with the network, or the database can somehow become corrupted. Step 1 Make sure you are up to date with the latest wallet software version. If in doubt, download the latest software from https://www.xtrabytes.global/#wallet. See the tutorial How to update your wallet to the latest version (Windows) for more detailed instructions. Make sure the wallet software is closed before proceeding. Step 2 Locate your wallets data directory. This is where the blockchain and ‘wallet.dat’ is stored. If you followed the guide ‘How to install the latest wallet software (Windows) you have the blockchain stored in the data folder. If you did not go through the steps to set a custom data folder. The default path is ‘C:\Users\your_windows_username\AppData\Roaming\xtrabytes’ Or do the following Hold the windows button and press R In the box, write %appdata% then press enter Go to Roaming Go to Xtrabytes Step 3 Now select all the files and folder, except ‘wallet.dat’ and delete them. It’s very important that you leave ‘wallet.dat’ in the directory, it contains your coins. Step 4 Restart your wallet software and the synchronization process will start from scratch. Credits: molten
For Mac users if you can't find your library try this : https://www.wikihow.com/Find-the-Library-Folder-on-a-Mac
After completing these steps, is there any reason to backup wallet.dat again, or is a backup of wallet.dat on a USB from months ago okay as a backup forever?