This is common after updating Portainer versions, and is usually caused by a cached page in your browser.
Step 1 - Find out if caching is the problem: Load Portainer in private/incognito mode. If it works then it means that your browser has cached the page.
Step 2 - Fix it: Clear your cache through the browser settings. If that doesn't work, check that the latest Portainer version installed successfully. If that doesn't work, log a bug report.