Inform your users what is happening by writing good and useful error messages. This will hopefully lead to few support errands in the long run as the user can understand what went wrong.
Write using plain words. Avoid technical jargon and error codes
Tell the user exactly what went wrong. What happened, and why?
Tell the user how the problem can be fixed. When will it be fixed and How can the user respond to the error?
Use we when the error is on the developer's side, e.g. "We are down for maintenance". Use you when the error is on the user's side, e.g. "Access Denied. You don not have permission to view this page"
Use words like "Page not found. The URL might be incorrect." instead of writing "404 Not found"
We don't always know how long an error will persist. Add links to a status page (if available), to a [[Twitter]] acccount, or another way for the user to get in touch if they need to.