No boot disk found

I'm using a Mac mini,

OSX mountain lion or mavericks ( not sure if it's been updated)

Using apple wireless keyboard and mouse

Backup end up to external HDD using Time Machine

I recently purchased a seagate wireless plus 1TB hard drive. During its setup I needed to reveal some hidden files in its media library, so I research how to do it on google and used 'Terminal' to display hidden files in finder. I used the command -

"defaults write com.apple.finder AppleShowAllFiles TRUE"

After doing this I restarted my Mac mini in the normal way. I did not use the KillFinder command.

The computer powered up

Gave a power up tone

Displayed a blank grey screen for around 30-40 seconds

Displayed a black screen with the message " error loading operating system "

I powered down the computer by holding the power button, removed USB peripherals including HDD used as 'Time Machine' backup then restarted to be greater with-

A power up tone

A blank grey screen for 30-40 seconds

The a black screen with a message " no operating system found -- insert boot disk and press any key"

After a online chat and phone call from apple i'm no further on. I've been told that I need to go to an apple store or an authorised repairer.

I tried starting up holding the option key,

"""" the shift key

"""" the Command + R

With no joy

Can anybody advise ??