If one or multiple folders and files are missing from the OneDrive folder, you may be using selective sync, and which case, you may need to update your settings to make those files available through File Explorer.
To change your sync folder settings in OneDrive, use these steps:
1. Click the OneDrive button in the bottom-right corner.
2. Click the More option.
3. Click the Settings option.
4. Click the Account tab.
5. Click the Choose folder button.
6. Select the Make all files available option.
Quick tip: When syncing folders, you're simply making the folder available through File Explorer, and it's not until you open the file that it'll download on your computer. So, syncing all your folders won't eat up your local storage.
7. Click the OK button.
Once you complete the steps, you should now be able to access and sync all your files and folders using File Explorer.