Sometimes there are XTensions which make files unreadable by versions of Quark without that XT. One notorious one is a Scitex XT, but if you drag the file onto an app called KillScitex (you should be able to download it somewhere), it makes it openable again. You do really need to give more detailed, umm, details of your setup when you ask for help tho.
MacBookPro C2D/2.16 2048Mb 120Gb OS10.5
PowerMac G4/867 1152Mb 440Gb OS10.4.10 ProSpeakers iSub USB2.0 S-ATA BT DVD±RW Zip250!
Custom Black&Platinum PowerBook G3/300 384Mb 40Gb OS10.2.8 FW
Epson 1290, Canon 20D, 300D, Sigma 70-200 EX f2.8, Sigma 2X EX, W800i