Apps and Settings gone

Hello, I ' ve bought a MAC a week ago and I`ve been through a strange issue here:

All my settings and apps are gone. It feels like it has been formatted, though I have all my downloaded files in the download folder.

What's going on?

Obs: I've been out for a couple of days and my teenager sisters have been using the computder. I hope they didn't mess it up.

Thanks!

 

as far as I know there is just one registered account in this computer, which is mine.

i`m very annoyed cause i had installed a lot of apps ,and theyre just gone. interstingly, the wallpaper i had set is still there.

i really wouldnt like to do this all over again, and imagine that I would have to do it once again and then again - no not possible.

I want to understand what is going on.

It sounds really stupid. I feel like a monkey in front of this computer , I `ve been using a PC for 15 years and I can handle it very well.

thanks a lot in advance!!!