Formatting builds or rebuilds the FAT (File Allocation Table) but does not physically delete images although the process of formatting might overwrite some image files. As to why the PC won't recognised the card is another matter for which hopefully someone with more knowledge in the area of computers might be able to help. In the meanwhile I would suggest trying the card in another computer or even in another PC. It could of course be the case that your PC card reader is not capable of reading SD cards especially the high capacity type which is something that does pop up on the forums from time to time.
Assuming that you get the card to be recognised on another computer (especially one that you know can read SD/SDHC cards then you might want to run Zero Assumption on it. This is probably one of the best pieces of data recovery applications available and what's more it's freeware.
http://www.z-a-recovery.com/