A user started to work remotely.

IP ranges are settings that restrict login access to Salesforce from specific IP addresses. Administrators can set IP ranges at the org level or at the profile level. If a user tries to log in from an IP address that is outside the allowed range for their profile or their org, they will not be able to access Salesforce. Therefore, if a user started to work remotely and is having an issue logging in, it could be because they are not in the IP range for their profile or their org. References: https://help.salesforce.com/s/articleView? id=sf.security_networkaccess.htm&type=5