Some of my photos in olympus master just disappear,why? Can I have the photos recover?

You cannot find anything on this digital camera?
Based on my knowledge, your problem could be resolve in these three ways:
1). Try to unhide these files.
According to your description, your files may be hidden anyway. Try to show them all up again:
Start => Run => Type in "control folders" and press "OK" =>Click "View" and hook "Show hidden files and folders"=> Press "OK"
2). Scan your computer with antivirus tool
Sometimes, the virus also would hide the files make them invisible for you. So, just try to do antivirus scan on your computer. (You should take care not to delete some needed files in the process.)
3). Restore your files back by using a digital camera data recovery freeware.
This problem also could be caused by the drive problems. So, if you want to save these files, you'd better try to use a digital camera data recovery freeware that always could help retrieve the inaccessible files back for free:
http://www.freeware-fix.blogspot.com/2013/02/photo-recovery.html

Good luck!

Card Data Recovery is comprehensiverecovery software for any Digital Media that allows users to recover deletedand lost photos, music and video files from basic format to high-end raw imageformats for most popular professional digital photo camera like Canon, Nikon, Sony,Olympus and Minolta which includesJPG,JPEG,TIF,TIFF,PNG,BMP,GIF,CRW,CR2,NEF,SR2,SRF,ORF,MRW. It also supportsvideo & music formats including AVI, MOV, MPG, MP4, MP3, WAV, and more

In this case you should try some photo recovery software to see if you can recover the lost photos, here are some programs you may rely on. Both programs can also recover videos and music.

Photo Recovery Software (for Windows)
Mac Photo Recovery Software (for Mac OS X)

Note: Before your pictures are recovered, do not attempt to save more files to the Olympus memory card in case the original files(your photos) are overwritten.