Half my iPhone contacts just disappeared

If the missing ones were Gmail contacts, Google Contacts on a computer has an undo changes option in the settings gear. It can roll your contacts back up to 30 days. Pick a date before they vanished and they sync back to the phone. Which account were they saved under, do you know?

When it's exactly 'half' it's almost always an account toggle, not deletion. Contacts can live in iCloud, Gmail, Outlook etc, and if one of those gets switched off or signed out, its contacts vanish from the app. Check Settings > Contacts > Accounts, go into each account and make sure Contacts is on. Then in the Contacts app tap Lists at the top left and pick All Contacts, sometimes it's just showing one list. If they were in iCloud and really are gone, sign into the iCloud website on a computer, open account settings and look for Data Recovery, there's a Restore Contacts option with snapshots from the last few weeks.