It sounds like your phone was set up to sync your google contacts. Under settings for your google account there are a list of different 'syncs' that you can choose to enable/disable. If you have your contacts enabled to sync with google, it will pull any of your contacts in from gmail (as you learned). Unfortunately, because it is a 'sync' that means it syncs both ways. If you delete a contact from your phone it will be deleted from your google account (ie gmail) as well. When you deleted all of your contacts on your phone you deleted them all from your gmail as well.
Luckily for you....Google has your back!
https://support.google.com/mail/answer/1069522?hl=en
Log into gmail in your browser. Click on the word "Gmail" in the upper left in red....select 'Contacts'. Go over to the tab "More" and select "Restore Contacts". You are then given the option of when you want to restore from (an hour ago, yesterday, a week, or choose a custom date to go back to.)
Once you've restored your contacts, if you don't want them to syn to your phone, make sure you go into the settings menu, google account, and make sure to de-select any of the sync services you don't want to use. I believe they default to sync.
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