Connect the camera via FireWire with it powered down. Then turn it on using the VTR or VCR mode. In the View Menu select Refresh A/V devices. Select the easy setup for HDV that matches what you shot. If this fails, try iMovie, and if that fails, try another FW cable. Sometimes just trashing preferences and restarting the mac with the camera connected and powered up brings it all back too.
If all the above doesn’t work. You have a hardware or software install problem somewhere. FW ports are touchy and don’t like powered up drives or cameras/decks connected to them NEVER do this.
There’s a PMU switch you can push on your motherboard too to reset the power management system wide to see if that’s the problem. Must have your computer unplugged from power first though.
If all of this fails… you might try reinstalling QT 7 using this method: https://docs.info.apple.com/article.html?artnum=301852 and if that doesn’t work… you have a hardware problem most likely… hopefully it’s not the camera’s port… that’s way expensive to fix. The mac you have is a tower or an iMac? If an imac that’s a new motherboard waiting to happen… can’t add a FW card to it like a tower could.
Jerry