I had the exact same problem with a Sandisk 32gb Class 4 card - died after 2-3 months of use. Luckily, I was able to back it up on my PC before it finally gave out. The guy I ordered it from is exchanging it, but I'm concerned that the same thing will happen again.
EDIT: Looking at one of the reviews for an SD card on Amazon, a review has offered some great suggestions to check the SD card is authentic and can perform up to standard (i.e., if labeled a Class 10, reads/writes at 10MBs/sec). Full review can be found
here.
One program is an app called SD Tools by Veluscek Ales (available
here). It verifies the card information and helps to determine if it's fake or not.
Another free program for the PC is called H2testw (most current version as of writing is V1.4), and this tool will perform read/write tests on the card.