Assuming this is not a permanent hardware related error, you can try to remove the read only via DiskPart
(assuming you are using Windows to access the card, on a Linux system you could use something like parted
):
after selecting the appropriate "disk" (in your case your SD card), type Attributes Disk
this will show you the cards attributes
if it show to be read only you normally use the command "attributes disk clears read-only" to remove the read only attribute
This should but your card back in writeable mode, good luck.
Otherwise I would just inform the store that you can not erase your data, some stores give you the option to destroy the card and give you an replacement, but make sure to ask first.