Start by installing Zotero 7 from the download page.
If you still have trouble, restart in Troubleshooting Mode (Help → "Restart in Troubleshooting Mode…”), which temporarily disables all plugins. If that fixes it, you'll need to identify the plugin that's causing it and then report the problem to the plugin's developer.
Upgrade Storage
If you still have trouble, restart in Troubleshooting Mode (Help → "Restart in Troubleshooting Mode…”), which temporarily disables all plugins. If that fixes it, you'll need to identify the plugin that's causing it and then report the problem to the plugin's developer.