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When the controller of a flash medium detects a problem it often **goes read only**.  This is a defence mechanism so that it preserves your data and prevents any further deterioration in the flash by further writes.  For example, the controller may have decided the flash has reached the end of its write lifetime and the flash can no longer be relied upon to store data correctly, so it prevents further writing.

In this situation the devices appears to be accessible as normal, but any writes are ignored.  This can cause filesystems to become confused, since the filesystem thinks a write has succeeded but in fact the data is unchanged.  Depending on how data is cached, the filesystem may have either the old data (which is on the medium) or an updated version in cache, which will be replaced by the old data when it is evicted from cache.  This can be the source of strange behaviour and sometimes machine freezes.

If the card goes this way, it's life is over.  The thing to do is copy the files off it onto new media while you can.  Then you can only throw the card away - if the data is sensitive, physically destroy it (cut it up) first.