So ive had this issue before and now my friend is having it. We moved his sound card, because he had it really close to his 8800 and we wanted to give it more breathing room, but once we installed it in the new slot, it stopped picking it up, and now it is not picking it up in any slot. The motherboard is a MSI NF4 ultra Platinum. He has an 8800GTS 640, and the sound card is an X-Fi. Any ideas what causes this, like i said, ive had this issue before with a different sound blaster, and its annoying.
 

Go into device manager and uninstall it there. Then do a 'Scan for hardware changes' and it should re-detect it.

You probably have an irq conflict.
You may be able to resolve it in the Bios or in windows device manager.
Otherwise I would leave the card in the old slot.

Of course you could try uninstalling and reinstalling the drivers.