Nintendo has released an official statement through the Nintendo Japan Support and Nintendo Europe Support pages confirming that over 160,000 Nintendo Network ID accounts have been affected by recent hacking attempts. At this time, hackers have have obtained private date such as birthdays, names, e-mail addresses, and countries. As it stands, credit cards and PayPal information have remained unaffected. However, if you share the same password for your Nintendo Network ID with your Nintendo account, your account may have been used to make unauthorized purchases. [Thanks, Nibel!]
In response to recent incidents related to some Nintendo Accounts, it is no longer possible to sign into a Nintendo Account using a Nintendo Network ID. We apologise for any inconvenience caused. Please visit our Support website for more information: https://t.co/NWyXLiS1wR
— Nintendo of Europe (@NintendoEurope) April 24, 2020
Nintendo mentioned at the start of the statement that hacking attempts were made as early as the start of April through the Nintendo Network ID system. Due to this, Nintendo has removed the ability to log-in to your account by using the Nintendo Network ID system. The Nintendo ID system was originally used for the 3DS and WiiU. These accounts are separate from the Nintendo accounts used on Switch. However, they accounts could be linked before the incident.
If your account has been compromised, Nintendo will send you an e-mail informing you of the situation regarding your account. So it is suggested that if you do have one that you check the e-mail associated with your account. Regarding unauthorized purchases, if your account has made any purchase without your knowledge, you can send Nintendo Support an inquiry and they will immediately cancel the purchase.
Nintendo recommends that all users set up 2-factor authentication in light of this breach.