Load up Aperture this evening and it come up with something like 'Get started with Aperture' 'would I like to turn on photo stream' and so on.
Please explain a little more: Has Aperture been running on this machine previously? And what exactly have you been doing before this happened? Did you install something new? Upgraded Aperture? Imported images?
Where is your Aperture Library? On your system drive or an external Volume?
And what is your Aperture version and your MacOS Version? You are writing Mt.Lion, but your profile says 10.4.11 - are you using 10.8.2?
The good thing is that the library and the application are two separate things. So even if the application 'reset' to a fresh install your libraries will still be there.
Sounds like the Aperture preference file got deleted somehow. What were you doing before this happened? were you doing anything with your ~/Library folder? Do any other apps behave as if they were just installed?
I'm using version 3.4.1. On 'About my mac' it says 10.8.2 which I think is most up to date. As I say computer working fine (although maybe not - the new imac is tempting...)
I wasnt doing anything (honest!) Just using the internet and general work.
Everything else is all as it should be, all documents in Pages, MP3s in itunes and so on.
So, only option is to reload all the photos, little cheesed off really, but oh well.
Hold down the options-key ⌥ while double clicking the Aperture icon in the Dock or the Applications folder.
This will bring up a panel to select the Aperture library to open. Do you see more than one library?
Then try each of the libraries in the panel in turn and see if one of them contains your photos.