Sounds like you didn't do the installation safely with regard to the old OS. The 64-bit one may have been using a 64-bit bootloader? In any case, you may have clobbered the boot partition of the old installation. You can try using bcdedit to add a boot entry for the old OS - it might still work - but you may need top use 64-bit Win8.1 recovery media instead (and doing so you risk clobbering the boot partition for the 32-bit install).
... Do I dare ask why somebody would *do* this? I mean, I could understand if you were installing Win7 or something, and even vaguely understand why you'd install 32-bit instead of 64-bit (though these days I pretty much exclusively use virtual machines for 32-bit systems) but why in the world install 32-bit Win8 on a 64-bit Win8.1 machine??