From today's website:
It's the last paragraph that left me confusedOverhead wire problems are causing disruption near Gordon Hill.
Because of this, there are delays of up to 45 minutes between Alexandra Palace and Hertford North. These delays are expected to last until approximately 15:30.
Passengers are advised where possible to travel to Stevenage and board a southbound service via Welwyn Garden City back along the Hertford branch to complete your journey.