Here are your options if a complete refresh does not resolve the issue:
1. Do a complete reinstall of WordPress.
2. Check with your web host to ensure something isn't wonky. It could be that you both are on the same web host and the host is having the problems.
3. Check with your web host to run a virus/malware scan on your server to ensure it hasn't been hacked.
If these things don't work, then more aggressive attempts could be made such as using FTP to rename your Plugin directory and seeing if this resolves the issue due a Plugin issue, changing WordPress Themes (some Themes today hook into the WordPress Administration Panels and can cause errors), or something even more drastic. Let's keep it simple to begin with.
I've seen this many times over the past few years and it has always been a browser issue.
Depending upon your browser, to clear the cache, go to the main menu of your browser and look for Tools or Options and choose Privacy or Security or something that offers you the option to clear cookies or history. Try removing history first, as that might do it, and then cookies.