problem with saving as png file

I don't know what i did, but whenever I want to save a file as a png in photoshop, it's there but when i go to upload it on imageshack, etc, it won't upload. I checked the properties and it said 0 bytes. So basically, photoshop isn't saving anything. It was working fine earlier today, then I guess i did something and something changed.

Usually a little box appears after I click save and i don't remember what it says but it doesn't appear anymore. Can anybody help?? D:
Thank you!!