If your employer was not required to notify you (and I don't know that), the person who garnished your pay certainly was required to do so.
I would contact the court or agency that issued the garnishment order and determine what notice, if any, was supposedly given to you. If a judgment creditor falsely claimed to have notified you of a garnishment, you can recover from that creditor.
Please be aware that I may not be licensed to practice law in your jurisdiction, and therefore am not giving you "legal advice". You understand that an attorney/client relationship is not to be established.