You've stated that you've tried the card in different card readers but have you been using different computers or the same one?
On around half a dozen occasions I've found that I couldn't get an SD card to open on one computer (irrespective of whether I used the built-in SD card reader or a USB one) but trying it on another computer instantly got it recognised and, perhaps surprisingly, it was then recognised again by the first computer without any problems.
I've mentioned my experiences several times here on AB to others who've had problems with SD cards and, in every case, my 'solution' (if that's what it can really be called) has worked for them too.
If that doesn't work, try this:
Insert the card into a card reader (preferably an internal one).
Click the Start button.
Click on Disk Management.
Hopefully you'll then see the card listed as a drive there.
Right-click on it and select the option to change the drive's letter.
Change the letter to something that you're not likely to use (e.g. from 'E' to 'M')
Close Disk Management.
The card should then be available from 'Computer' as usual.