I just had this same issue happen to me a few weeks back. My mico SD inexplicably unmounted from my phone, tried to plug it into my computer using my SD adapter and it acted as if nothing at all was plugged in. I did a google search and found some recommended programs to try and retrieve data from corrupted SD cards, but nothing worked since the computer wouldn't even recognize anything was plugged in. I tried plugging it into my netbook, my Mac computer at work, one of those photo lab machines at Target--nothing would recognize there was a card plugged in. I was distraught because I hadn't thought to backup my photos in months, and I did not want to lose them.
However in my searching for a solution, I saw many people mention Recoverfab (recoverfab.com), a professional in Germany who has a good success rate at recovering data from these SD cards that won't mount. I was sold the minute I saw that if he is not able to recover your data, there is no charge. I contacted him and filled out the order form. Shipping from the midwest, my card got to him in little over a week and he had my photos recovered the very next day. Depending on the size of the card it can get a little pricey, but for me it was well worth getting all my pictures back. I'm going to be sure to set my google+ account to backup my pictures going forward now, but if I ever had something like this happen again I would use Recoverfab again without hesitation.
I hope this helps, and good luck on retrieving your data!