Ransomware can be initiated via a spear phishing attack. At first, they will attempt to trick a user into providing their e-mail credentials (password reset) to a fake website (often looks like Office 365 if that's the provider per MX records). They're given an error, and soon ignore it. Meanwhile, the criminal logs in, creates e-mail rules and a forwarder to an external GMAIL account or whatever. Now that they have full access to the account (including a copy of the outbound e-mail signature), they can proceed to spam all contacts with fake documents containing ransomware in an embedded macro.
the malware proceed to crawl on every share and encrypts the files. Most of the time, on a Friday or weekend after 5pm so as to have as much time as possible to 0wn the files without much suspicion until it's too late.
In most cases, I can restore all the data via shadow copy (assuming the quota was large enough on the volume to begin with), local backup, or from the cloud. Typically servers themselves never get hacked, only the shares, as the executable can't run on the OS of said server. The only time a server itself gets hacked is if a user account is a member of the Domain Admin group in AD - and that's because the previous IT admin was incompetent to be granting that to users in the first place.
Remember, Ransomware as I know can only encrypt enumerated volumes; be it mapped drives or direct attached storage. Perhaps the newer versions will crawl the network and attempt damage to whatever the user has access too. In any case, if you must backup a local machine, it's best to do it over the network where the backup network credentials are initiated and stored in the backup application, and not with an existing user account. Meaning, you don't want want the Ransomware to take out your backups as well, because if it has access to IT WILL TAKE THEM OUT (along with purging shadow copies via the VSSADMIN command) . After all, it wants the ransom payout. If you have access to backups, there's no need for that. The malware authors know that.