Video and audio not appearing or playing in source window or timeline.

I'm trying to do a quick fix to an .mp4 file, but I'm unable to get anything to play within the app.

I imported my .mp4, but when I double-click or drag it to the source window, nothing appears. I can't play it, see video or hear audio, and the timecode doesn't change when I scrub through. The thumbnail of the video sometimes appears, but sometimes doesn't.

When I drag the video straight from the project folder to the sequence, the video and audio tracks appear, but again there's no video or audio showing up in the program window.

I thought the problem might just come from .mp4 files, but when I imported an .mp3, I got a similar result. This time, the graphic of the waveform shows up in the source window, but no playback occurs after pressing space or clicking the play button.

And like with the .mp4, I can drag the audio down to the timeline, but I can't play it.

I also tried this with a .mov file, but no luck either.

The past few times I've tried to quit the app, it goes unresponsive and I have to Force Quit. Maybe there's a larger issue?

Is there something that I'm missing, or a setting I need to find? Any help at all is appreciated. The app is unusable with this current problem.