[support] Strange Drupal 7 problem, site wants to reinstall

[Roger]

I have a D7 site live and working.
I copied the database and /sites folder and did a fresh install  of 
D7.23 to my pc about a month ago.
Using that on my Fedora 19 desktop version for development ideas, all 
worked well and I was doing some dev work till late last night.

Today I cannot open that Drupal  localhost/mydev .
I have a localhost/sandbox site which does work. All file ownerships and 
permissions are the same on sandbox and mydev.
To use, localhost/mydev wants to do a fresh installation.
/sites/default/settings.php is the same as it was last night but even 
after chmod 777 settings.php, install reports that it does not exist.
/sites/default/files is the same as last night but it reports that 
/sites/default/files does not exist.

I suspect that it may be an ownership problem where /files and settings 
have to be owned by apache but have not had this issue on other test dev 
sites, or maybe apache has calked it again but that does  not explain 
the /sandbox site working.

Can someone please advise me on what to look for and what permissions 
should apply to appropriate files, perhaps even suggest why things may 
have changed between last night and this afternoon.

Most curious
Thanks in advance
Roger