This has been happening a lot. Both my wife and I had it happen less than a week apart. What you have to do is call in to customer service and ask to have your voicemail turned off and then back on (basically they are resetting it. You can't do it yourself, only Boost).
The first time I did it, the gal said it was done but it still didn't work (accent sounded like India). I called the next day and got a guy who knew his stuff and he had me fixed up in a couple of minutes on both phones. They also gave me a $5 credit for my inconvenience. You will have to re-record your greeting and set up a password for your voicemail again.