Have you tried restarting the phone? This often will fix the issue.
Also, have you recently (prior to the problem starting) installed any new apps or updated apps? The change could have effected the card. If this is the case, you could try uninstalling whatever was recently installed to see if it fixes the issue. Restart the phone after and uninstall.
Did you remove the card, while the phone was turned on? This can cause errors, and even corrupt the card and/or the data on the card. If that is the case, there may be nothing you can do to get the card working again, without loss of data. While I card may be fixable, or reformattable, that would still result is loss of data.
Also, if possible, I would try and connect the card to a computer and see if you can read it there, or from another device. NOTE: Make sure you do not let it be formatted to that computer or device. You are just trying to see if they will read it. If they do, then you know the problem is with the phone.