I can't see my preview like I can only see black

[Katie5E67]

So i'm not sure what's going on or what's even the problem. A few days ago After effects was working just fine but then yesterday when I was putting in the clips for an edit that I was gonna make, I played it and I could only see black. Like a black screen and my clips were there but for some reason it wasn't showing. My timeline wasn't loading at all either. I watched a lot of videos on youtube but none of them were really helpful for my problem. I'm not sure if the timeline is the problem or the preview or anything else but I really need help!! Please Help!! I want to fix this as soon as possible. 

[Anna Lander]

a pair of stupid questions 🙂
did you add any effects? may it be that Opacity is occasionally set to zero?

[Katie5E67]

I didn't add any effects since I couldn't see any of my clips and I chacked my opacity and it says 100%