Photoshop freezing

I've had an issue with my images suddenly freezing while I'm editing them.  The 'ruler' along the side also goes blank, sometimes both rulers and sometimes just the vertical one.

When this happens, no shortcut commands work and my cursor is just the black arrow, reglardless of which tool I select.

If I go onto a different image, however, photoshop works just fine, so it is not as though it is frozen as an entire programme. 

The only fix is to save the image down and reopen, which is quite frustrating.  Any ideas?