Start with what you see

Steam starts without Millennium

Close Steam, run millennium diag, then run millennium doctor if the hook check fails.

An update did not finish

Keep Steam closed. Run the diagnostic command, then retry millennium upgrade.

A theme or plugin causes trouble

Run millennium config plugins, disable the recent change, save, and restart Steam.

You need to share a report

millennium diag --share

Review the sanitized report before sharing its link.