It depends on where the contacts are synced.

Google contacts (Android): open contacts.google.com on a computer, click the gear, then Undo changes, and select a time range like the last 10 minutes, hour, day or week, or custom up to 30 days back. Contacts restore on the phone after a sync.

iPhone with iCloud: sign in at icloud.com, open Account Settings, and under Advanced choose Restore Contacts. Select an archive from before the deletion. iCloud saves the current contacts as an archive before it restores, so you can undo.

Local contacts that never synced can only be restored from a phone backup or an exported VCF file.

Do it soon, since restore points exist only for a limited time.