Do you have any FX added to either the timeline or clip? Also, by any chance did you mute the video track (the 'M' in VMS 14 in the track header)?

What model of camcorder did you shoot this video with and what format and resolution is the video?

Does the video look dark when you play it in, say, VLC Media Player?

Steve: I've used a Sony Handycam HDR-FX7 for several years now.  I was having no issues until I finished.  Until then, I was able to view it on the preview screen as I edited.  Since it's my first project using the "new" Movie Studio, I'm still getting used to where everything is and I suspect I inadvertently clicked something.  What that is, I don't know.  As stated, it renders dark as well so it's dark when I view it in any other media player.  I will have to correct myself, I did one 30 second project before this and it was fine. 

Do you have any FX added to either the timeline or clip? Also, by any chance did you mute the video track (the 'M' in VMS 14 in the track header)?

I have plenty of FX added (fade in/outs and old film FX primarily.)  And I do not have the video mute button activated. 

...and I do see the video in the small screen when I want to "open in trimmer."

@tom-p

Some other ideas...

1- Do you have more than one track in parallel? If so, what happens if you mute the top video clip?

I have a feeling this next one is it....

2- In the track header, right under the M(ute) icon, you'll see an icon that looks like an '@' over a filled circle. This is the compositing mode. Click on that. By any chance did you unintentionally flip it to 'Multiply (mask)? If so, switch it to 'Source alpha'.

@tom-p

Some other ideas...

1- Do you have more than one track in parallel? If so, what happens if you mute the top video clip?

I have a feeling this next one is it....

2- In the track header, right under the M(ute) icon, you'll see an icon that looks like an '@' over a filled circle. This is the compositing mode. Click on that. By any chance did you unintentionally flip it to 'Multiply (mask)? If so, switch it to 'Source alpha'.

Negative on the result, sorry.  Yes, the alpha button is set. 

@tom-p

Ugh... I was almost sure of it!

Multiple tracks? Can you post a screen shot of the Vegas screen (and preview screen)? I'm still going on the premise that something was clicked that shouldn't have been clicked....

Wait...

In your preview screen, there are multiple icons. From the left, there is a gear, monitor, fx, and the splitscreen view. The last one is a new addition to VMS14. Click the splitscreen preview icon. I was initially thinking about it (other users have had issues with clicking it by mistake and seeing strange behaviour). If it is on, the icon goes black (hard to see from the default grey). It should have a grey background. If that works, it means that you may have selected it (and possibly 'clipboard' in the menu). Hoping for a positive result here!

Sorry I've been away, in more ways than one...

Even when I drop a "new" file into the existing timeline, it's dark.  More importantly, when I create a new project using recommended settings, I drop a completely different video file into the timeline and then....dark.  Grrr....  I'm getting the feeling I should just uninstall and reinstall, making sure I don't delete the video files.