Welcome to the forum!
You can find out where Inkscape is trying to save it, by looking at the path in the field that is called "Filename" which is just beside the button which is now called "Export As" (in the new stable 0.92), in the Export PNG dialog. If you click that button, you can change to wherever you want to save it.
By default, I think it tries to save in the user directory, which in Windows is C:\Users\username\bitmap.png. If you don't change the name, Inkscape will call it "bitmap.png"
Or here's something that I find myself doing sometimes. I'll click the Export As button, and change the name and location, and click Save, and then I can't find the image. Well, it's just that I wasn't paying attention, and thought I was finished after clicking Save, when really, I needed to still click Export button.
If you still can't fix it, you could try this. I couldn't tell how many times I've solved a problem, while I was posting a message to ask for help. That's because I think I should post the steps I'm taking, so people can understand what I'm doing. And while I'm pragmatically putting down every step (I do the step, then stop to write it down, then do the next step, and then write it down) I figured out what was going wrong.
Well, maybe that's just me. Another trick is to save the SVG file, and sleep on it. Fresh eyes are usually better.
If none of that helps, then all I can do is ask you to write down the specific steps you're taking. Also your Inkscape version and operating system.
Good luck
