It’s possible installing themes or plugins can cause compatibility issues between your WordPress, theme, and/or plugins.
I’d recommend checking the theme/plugin is compatible with your version of WordPress, that all the required plugins are installed (if any) and that you have the supported versions of such plugins. The requirements of a theme or a plugin are typically found on the website where you purchased or downloaded it.
I’ve included details on two plugins below which are very helpful in quickly rolling back and forward WordPress and plugin versions.
You can check your current version of WordPress by logging into your WordPress admin area, navigating to the dashboard. Here you should be able to find the current version of WordPress you have installed, either in the “At a Glance” section or at the bottom of the page (depending on the version of WordPress you have installed).
If you need to downgrade your WordPress to an older version then you are able to achieve this by using the “WP Downgrade” plugin.
Once installed you will be able to navigate to this plugin from the menu in your WordPress admin area. You will be able to select a version of WordPress and then update the core WordPress files and database to your selected version.
You can check the versions of your plugins by clicking “Plugins” on the menu in your WordPress admin area and selecting “Installed Plugins”, next to each plugin you should be able to find the installed version.
If you need to rollback to a previous version of a plugin then you are able to achieve this by using the “WP Rollback” plugin. This plugin will add a further option “Rollback” next to each plugin on the Plugins page, when clicking on this option you will be prompted to select a version of the plugin and then you can roll it back (or forward if you need to upgrade a plugin but not to the latest version).
You can find and install these plugins, both of which are free at the time of this post, by searching for them from the “Plugins” page within your WordPress admin area.
I hope this is helpful.