A mental health website struggled to cope with demand after a promotional video voiced by the dukes and duchesses of Cambridge and Sussex aired on TV.
The film screened on Sky, ITV, Channel 4, Channel 5 and MTV on Monday evening.
The Every Mind Matters website for a time was intermittently showing the message: "Something went wrong. Please refresh or try again later"
Public Health England said the crash may have been due to a surge in traffic but the website was now working.
The three-minute film is intended to promote Every Mind Matters, an initiative by Public Health England (PHE) and the NHS, to help people look after their mental health and support others.
The website went down for a short period within minutes of the advert being broadcast.
A PHE spokeswoman said: "We think it was due to high traffic. We had technicians working on it immediately and we're back up and running now."
The film is narrated by the Duke and Duchess of Cambridge and the Duke and Duchess of Sussex, who in May launched a text messaging service for people experiencing a mental health crisis through their royal foundation.
In the film, written by Richard Curtis and directed by Rankin, Prince William begins: "Everyone knows that feeling, when life gets on top of us.
"All over the country, millions of us face challenges to our mental health - at all ages - at all intensities, and for all sorts of reasons.
"We feel stressed, low, anxious, or have trouble sleeping. Me, you..."
Prince Harry continues: "Your brother, your mother, your colleague, or your neighbour. Waiting, wondering, hoping, hurting.
"We think there's nothing to be done. Nothing we can do about it."
Meghan then says: "But that's so wrong. There are things we can do. From today, there's a new way to help turn things around. Every Mind Matters will show you simple ways to look after your mental health."
Catherine continues: "It'll get you started with a free online plan designed to help you deal with stress, boost your mood, improve your sleep and feel more in control."
The royals are joined by other celebrities and public figures whose lives have been affected by poor mental health.
They include the actresses Gillian Anderson and Glenn Close, singer Professor Green, former England cricketer Andrew Flintoff, television presenter Davina McCall, and Bake Off star Nadiya Hussain.