I read that the ransomware petya looks like a chkdsk process
pic.twitter.com/IqwzWdlrX6
so if you don't have started a chkdsk task and this message appears, you should shut down your device immediately.

More advise:

1. Keep your device and virus software up to date all the time (security patches and updates)
2. Backup your files!!!!!!
3. Don't use global network user (one user for all network drives) and never use an administrator account for it.

If you think you're infected, shut down your device. When your device is turned off, no files can be crypted.

Helpful:
https://blog.kaspersky.com/new-ransomware-epidemics/17314/