.MOV Quick Time movies downloaded with Yosemite Update, why can I no longer play them on my iMac?

Please help me! I have been sifting through tech forums and apple support sites for days. I have downloaded nearly a dozen applications including Wondershare Video converter and MPEG Streamclip and MediaInfo trying to convert the 26 files from a major trip I just took to be used in iMovie but I can't even get them to play. It says they have a file size of zero bytes but I cannot figure out why? They were downloaded onto my sister's MacAir and she put them in her dropbox so I could save them to a folder on my iMac desktop. She unfortunately deleted all of the original videos from the camera and I desperately need to make these seemingly now empty files work on my computer. I'm not great with tech language but if anyone could please help me or just explain to me what happened to all of these videos I would really appreciate it.