Can you reproduce this in Troubleshooting Mode (Help → "Restart in Troubleshooting Mode…”), which temporarily disables all plugins?
You should also check your database integrity from the Advanced → Files and Folders pane of the Zotero settings.
If all your data is online, your best bet may be to 1) disable all plugins, 2) move the zotero.sqlite database in your Zotero data directory out of the way (e.g., to a backup location on your desktop), and 3) restart Zotero. It will then create a new database and pull down the data from your online library.
Upgrade Storage
You should also check your database integrity from the Advanced → Files and Folders pane of the Zotero settings.
If all your data is online, your best bet may be to 1) disable all plugins, 2) move the zotero.sqlite database in your Zotero data directory out of the way (e.g., to a backup location on your desktop), and 3) restart Zotero. It will then create a new database and pull down the data from your online library.