Storm asked did you check the SHA256 checksum of the downloaded .iso file against Zorin published value. That to confirm no corruption during download. Before you create the Zorin installation USB.
Useful advice for new installation here: Before you install
Assuming you check the SHA256 is good and your Zorin Live USB is good. Boot the Live USB into "Try Zorin" you should find Grub Repair (or Boot Repair). Run that ans see if it fixes your boot issue. See this No Grub, unexpected issue using boot repair - #2 by Aravisian