iMAC automatically deleted all my files with in 10 sec?

Hi,

So I have been using Apple products for 4 yrs now and never had a problem, today something strange happened as per below.

- I log in on my PC

- Randomly checking emails & listening to music and then decided to download and ftp client ,hence started downloading and just after 2 sec of download the entire iMac kind of freezes and I am unable to click anywhere.

- I tried force quit but that also didnt work for sometime and then I had to reset the iMac and when it started again all the data on iMac was erased including files on desktop/documents/my downloads.browser history was all cleared

- I still cant believe what I saw and was frustrated on loosing all important data, have out now on a recovery software mode..but its analysing data and will only show lost data after 4-5 hours

- how did this happen? did anyone remotely logged into my iMac and did this?

Can someone please help me, as I am a wedding photographer and i had a v.v.v important data on the Imac which I cant afford to lose.

I am using OSX EL Capitan -

please help