Many flash memory cards are designed to go into read-only mode if they fail. That way you can at least recover any data which is written to them. Depending on the failure, writing to the card could overwrite existing data. And continued writing could cause the card to fail entirely (can't even read from it). So they're designed to go into a read-only safe mode to give you a chance to recover your data. Unfortunately there's no hope for the card if this happens. It is dead and needs to be replaced.
First, try to figure out if this is what's happened to your card. Try the card on a computer and see if you can write to it there.
Try "low level" formatting the card on a computer. That's an archaic and inaccurate term in the...