Have you tried formatting the card? Put the player in MSC mode. With the card in the player, and the player connected to the pc, format the card as FAT 32 with 32K sectors.
Choose start, programs, accesories, command, prompt. Use the command
Format [driveletter]: /FS:FAT32 /A:32K
Driveletter is the drive assigned to the memory card. Make sure that you have this correct.
Formatting will delete everything on the card. It may make the card usable again though.
I don’t think your card is dead. I think what happened is that some files are on the card transferred to it in MTP mode. This may give you problems if you try using the card in MSC mode. Getting rid of the files on the card should then make the card usable. I had a card that my pc wouldn’t recognize when in adapter in the PC’s card slot. Then I remembered that the card had some files on it transferred to the card when in the player in MTP mode. I put the player into MTP mode, connected to the pc, deleted all the files on the card, then removed the card, put the player back in MSC mode and put the card into the adapter then into the pc card slot, and the card then worked fine with the pc.
If the problem with your card is something else, then formatting might also solve the problem. Of course formatting loses everything on the card. I am good about keeeping backups. I don’t just keep copies of what is on my players on my pc, I also have copies burned to DVD disks.