The first thing in a situation like this is to not write any new data to the partition on which you have lost data. Only mount your SD card read-only from now on.

Second, let me explain your situation in more detail. There are many programs which can recover this data. The data isn't truelly gone, its unlinked. Only once it is overwritten, the situation becomes much more complex. Therefore I recommend you to mount the device read-only.

Detach the SD card and insert it in a Windows device, and use GetDataBack for FAT32 to recover the data. There are also some Live CDs which allow one to recover data. It is possible to easily recover data from the popular filesystems Ext2/Ext3, FAT32, and NTFS.