Hi, welcome in this forum where users try to help other users.
I hope (because you gave no information) you are on Joomla! 3.9.1 and JEM 2.2.3.
Normally you should look for help, which you can find in Backend JEM control panel >help.
Because you changed a lot without knowing, it can be that you have to uninstall JEM, but to uninstall the JEM database too, you should first go to Backend >JEM>Control Panel> Settings>Global Parameters>Cleanup DB on Uninstall=Yes
Then uninstall all JEM. And Reinstall.
To understand the JEM system I propose you to use the protostar template. So go to template and activate protostar.
In backend JEM control panel you can choose load sampledata. Then look and consult the help to learn more about JEM.
After you can adapt for your template.
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