Can’t access site admin after 4.7 upgrade.

Try deactivating all of your plugins to explore whether any could be responsible. If none are, keep all plugins deactivated and switch to the Twenty Fifteen/ Fourteen theme to see whether it could an issue with your theme.

If you cannot access your dashboard, you can deactivate all of your plugins via FTP.

To deactivate all of your plugins via FTP, you just need to rename the “plugins” folder (yes the “plugins” one). Then WordPress will automatically deactivate all of your plugins. You can rename the folder name straight back afterwards.

If still that doesn’t work, you can try replacing the core WordPress files. Pay attention to the bits in bold:
Try downloading WordPress again and delete then replace your copies of everything except the wp-config.php file and the /wp-content/ directory with fresh copies from the download. This will effectively replace all of your core files without damaging your content and settings. Some uploaders tend to be unreliable when overwriting files, so don’t forget to delete the original files before replacing them.

I just had this same problem and it was resolved by deleting a plugin I tried to upgrade as well. In my case it was upgrading from Yoast 3.9 to 4.0 which caused it.

Hi Andrew,
Brown trouser moment resolved. Renamed plugins to pluginsold via ftp and this resulted in three errors from w3 total cache. Restored name for plugin directory and renamed the 3 php files mentioned in error messages. I then had a message to update database to latest version, crossed fingers and said yes.
All appears to be hunky dory now and new versions of the 3 php files appeared by magic.
Thanks very much for you guidance, much appreciated. BTW am I correct in thinking I can safely delete the 3 php files that I renamed?
Thanks again,
Eugene

If your site works fine with those files renamed then you can technically remove them, but I don’t know how all the plugins work so they may be useful in some other operation.