This is a malicious software that locks up computer data and encrypts it with unbreakable encryption. Then you are asked for a random sum of bitcoins (virtual currency) to decrypt it. It ATTACKS USER COMPUTERS AS WELL. Not only companies have been attacked.
Ransomware spreads through eternalblue vulnerability found in Microsoft Windows. Once it infects one computer it can quickly spread to all computer on the network. It tries no one but few options to infect a computer. It’s evolved variation of Wannacry with improved spreading capabilities. It spread through phishing mail campaign with malware attachments.
There is no clear instruction as yet what to do, but ransomware waits about one hour before restarting the computer. If you suspect that you are infected by it (or any other encryption virus) just don’t let the computer to load after the restart. Switch it off instantly. That way you will prevent Petya from encrypting your files and data can be saved. If you are not good with advanced computing we suggest calling a 24/7 computer technician or data recovery company.
Try to keep your windows system up to date. Petya security patch update will be installed with them. If you prefer not to get all the updates, you need to download patch from here and install into your computer.