All the titles and legends are not longer visible. What happened?

We only know of one time this has happened. That turned out to be a corrupt font. Restoring the fonts in Windows solved the problem. 

Another possible explanation would be if all the text were written as white on a dark background, but then the background color was changed to white. Try changing the background color of the page to see if the text returns.

A tip: Click Control-A while on a graph or layout to select all objects.  Even objects that are all spaces or whose color matches the page background will be selected. 

Also note that graph and axis titles can be turned off in the third tab of Format Axes dialog.