Old one but as I face the issue now, it's still actual.
1. What says Alabalcho make no sense. 1st, it require to know WHY it is read only. Manual or software action ? or spontaneous change ?
2. If software or manual change, there's many ways of getting back to read-write. Just google it
3. If the content of the card has been encrypted, it's another game
4. SD and micro-sd cards are not reliable ! I face it often. My latest experience: a brand new 128GB microSD. 1st days, I copied 75GB of music in it. Then I put it in my media player. works once. Then, files could not be read anymore. Put back in my Mac: It's read-only... I tried all the possible workaround, none worked. Windows, Mac OS X, Debian, MicroSD to SD Adapters (multiple), USB readers (multiple), no issue. I end up sending it back to the provider for replacement, as proposed by Transcend here: http://www.transcend-info.com/Support/FAQ-400
So if your SD or MicroSD card is all of a sudden read-only and you didn't take any action for it, and you're sure it's not a "Lock" accidently engaged on your adapter or your card, then don't lose time, and send make it replaced. Many of them have Lifetime Warranty.