Advanced (or adventurous) users can repair their databases without uploading them.
Be sure to make a backup of your entire Zotero data directory before performing these steps.
Then type the following commands in Terminal:
Your Zotero database should then be repaired, and Zotero shouldn't show any errors. Once you're sure of this, you can delete the zotero.sqlite.old file.
Post to the Zotero Forums with any problems.
Same as the macOS instructions, with appropriate modifications.