I lost all my data once I reboot my system. Was this not saved on Zotero? How did this happen? I thought Zotero is like an email where things are usually saved. Can I retrieve my lost data and citation bank?
It's not really clear what you mean here. The Zotero desktop app is a local program that saves all data locally on your computer. You can optionally set up syncing with Zotero servers.
If you were using the desktop app, your data would be present when you reopened the app unless you deleted it from outside the app (e.g., by using some tool to clean up disk space and deleting your database by accident). Simply rebooting wouldn't have any effect on your data.
Upgrade Storage
If you were using the desktop app, your data would be present when you reopened the app unless you deleted it from outside the app (e.g., by using some tool to clean up disk space and deleting your database by accident). Simply rebooting wouldn't have any effect on your data.
If you're not seeing data you expect, see Locating Missing Zotero Data.