I Can't see through my viewfinder and I've tried everything online.

I was taking alot of pictures on fourth of July, and right before we left I realized I couldn't see through my viewfinder anymore. It was black. I thought it had something to do with being outside in the wind and smoke from the fireworks. So I looked up different ways to fix it online, but none worked. When I slide over the bar that holds the lever in place that holds down the mirror doesn't do anything but the lever is all the way to the left. I can move the lever over so it will show the mirror, but it won't stay there.