We use VPNs to connect between datacentres. Please see GOV.UK and Virtual Private Networks (VPNs) for details of the VPNs we have and a description of the impact of each VPN failing.
If a VPN is down, it is likely caused by a network issue due to a vendor incident or our own misconfiguration. If no changes have been made recently to our network configuration, the best action is to contact the vendors involved. If the VPN connects one vendor to another, you should open a ticket with each vendor as the problem could lie on either side.