How can I get my photos back?
Does the camera allow you to connect it with the computer? Perhaps the storage is non-standard?
Run a program like PhotoRec against it, which is made exactly for this purpose.
Perhaps something changed on your computer, try another one.
Perhaps it is stored in Linux format? Try a random LiveCD (don't install, just work on it) to see if it helps.
If those things don't help, more advanced forensics are required. A computer expert (which I read as your local computer supplier) might or might not be able to recover it, it's worth trying but it could cost you some money depending on the situation.
How could this happen?
Did this work before? Then it might just be an incorrect way of accessing it.
My guess is that there has been a magnetic force near the SD, if not, it might've been a power bump?
Perhaps your SDHC card is getting old or used a lot and had it's time?