You'll need to say what the actual problem is that you're experiencing. See Steps to Reproduce for the kind of description we need.
In any case, start by updating to Zotero 7 and checking your database integrity from the Advanced → Files and Folders pane of the Zotero setting.
If the problem — whatever it is — still occurs, restart in Troubleshooting Mode (Help → "Restart in Troubleshooting Mode…”), which temporarily disables all plugins. If it still happens, provide a Debug ID (different from a Report ID) for reproducing it, along with an explanation of what exactly you're doing and what's happening.
Upgrade Storage
In any case, start by updating to Zotero 7 and checking your database integrity from the Advanced → Files and Folders pane of the Zotero setting.
If the problem — whatever it is — still occurs, restart in Troubleshooting Mode (Help → "Restart in Troubleshooting Mode…”), which temporarily disables all plugins. If it still happens, provide a Debug ID (different from a Report ID) for reproducing it, along with an explanation of what exactly you're doing and what's happening.