Try clearing your Web browser's cache and cookies and make sure it is on latest version. If the issue persists, try accessing Yahoo Mail in Incognito mode. If this works, check your extensions and remove those that are unnecessary.
If the issue persists, try with another Web browser. Check also your network connection - a slow connection may prevent Yahoo Mail from loading properly. Your antivirus may also interfere with the attachments - try adding an exception to allow you download attachments.
Let me know if that helps.