I've found the reason and the solution to this problem:
Since CS5, Adobe chose to not show the Save As.. window since its resolution is bigger than the native resolution of many netbooks (1024x600). That's why the Adobe CS5 minimum requirements specify the 1024x768 resolution too.
I've been using my Google-fu, and many solutions were only for specified netbooks (Acer, Nvidia/AMD related netbooks, and Dell), but this one in particular is for EVERY netbook that isn't capable of choosing a higher resolution than 1024x600:
- Go to your Desktop, Right click to the background and choose Properties.
- Enter to the Configuration tag → Advanced options button
- Untick the "hide the modes that this monitor can't show" option (My Windows is in Spanish, you may know what I mean :D).
- Apply, accept.
And there you go, now you can specify your resolution! (1024x768 in my case).
This is for people who find the page via googling, and they're having this issue on a NEWER version of Photoshop (in my case CC 2015.5)
In my case it wasn't the same problem as OP had (netbook resolution), it was apparently just out of memory. I have 100 undo levels and was using lots of history brush when it happened. I couldn't get a "save" or "save as" dialogue on any open document. Even new ones.
What DID work was File - Export As... - choose the default PNG. I got a 32-bit PNG with transparency so definitely no quality loss. I then restarted PS and everything's OK again. – CreeDorofl – 2017-10-05T20:18:34.297