Most likely one of your existing plugins is breaking it. You can try holding down Shift while starting Zotero to start it in Safe Mode, and then see if you can access the Add-ons pane.

If selecting Tools\Add-ons does not open that dialog as usual, it seems that is usually due to a Zotero process that has remained running after Zotero was last closed (I've never figured out any pattern as to why that happens sometimes; fortunately it's rare ... which I guess also makes it harder to troubleshoot). Close Zotero again, and (on Windows) use Task Manager to check if a Zotero process is still showing there even though it was supposed to be closed. If so, kill it. Then restart Zotero in the usual way, and Tools\Add-ons should work again. Or if you're not comfortable doing all that, just restart your computer.