A flash drive becoming read-only is usually a failsafe mode built into the drive by the manufacturer. The device no longer works (usually the pool of spare flash memory cells used to replace cells which failed has been exhausted), but it was programmed to "die" while still leaving your files readable so you could recover them.
The more reputable brands carry a lifetime warranty. Contact the manufacturer about doing a warranty exchange. If your brand does not have a lifetime warranty, I would suggest you buy from one which does next time. Also be wary that certain drives from unreliable sellers are bootleg and not genuine. I know Sandisk does not honor the warranty on counterfeit Sandisk flash memory purchased from eBay.