Most of our home / mobile users are utilising some kind of VPN to connect back in to their office to gain access to legacy systems that are not Cloud compatible. VPN connectivity is dependent on several elements all being in place including a working internet connection from the device, associated access software correctly configured, VPN permissions being set and any security tokens / groups being enabled as required, in addition to MFA being operational in some circumstances. It is always worth checking in the first instance, if you are in a public area, that the Network you are on permits connections to private VPN services as some will block this.  Typically most VPN issues can be resolved by resetting the credentials however – the locked account mentioned above can prevent access without your users realising.