To access safe mode go to:
http://your-site-name/safe-mode for example this is the safe mode for meta:
To exit safe mode simply close your browser tab
When entering safe mode you can select between:
- Disable all site customizations
- Disable unofficial plugins
- Disable official plugins
By default all 3 options are selected, but you can enter safe mode multiple times to isolate the culprit breaking your site.