If you deleted a large number of items from the bin, you might need to let it go for a while — potentially an hour or more — at startup. Once it finishes, it should start up quickly.

You can check Help → Report Errors… to make sure there's not some sort of "transaction timeout", which might indicate that it's stalled. Temporarily disabling auto-sync and restarting Zotero might help avoid that.

We're planning to optimize this in the future, but for now you may want to avoid emptying thousands of items from the trash at once.