Common causes & fixes:
• app.js or app.css failed to load (404): Make sure app.js, app.css and index.html are all in the SAME folder on your server.
• Backend errors: Open pos-backend/error_log in File Manager — the last lines name the exact problem.
• Old cached files: Press Ctrl+Shift+R (or Cmd+Shift+R on Mac) to hard-refresh.
• Take a screenshot of the text above and send it for a precise fix.