The acknowledgement
The first reply says you know about it, what is affected and that you are working on it. Do not guess at the cause. A short, plain message beats a long one with details you may have to take back.
The update
Promise a time for the next update and keep it, even if the news is that nothing has changed. Customers cope with delays better than with silence. Reuse the same wording each time so it is quick to send.
The resolved note
When it is fixed, say so plainly, say what customers may need to do, such as log in again, and thank them for their patience. Wait until you have checked the fix before you send it.
The follow-up
After a serious outage, send a short account of what happened and what you are changing. Keep it honest and specific. If you owe customers a credit, say what it is and how they get it.
Set it up so it is fast
Save the four replies as macros with the details to fill in marked clearly. Decide in advance who sends them, where customers can see status, and how you will tag outage tickets so you can reply to all of them at once.
What not to promise
Do not give a time for the fix unless an engineer has given you one. Do not say it will not happen again. Promise what you control: updates, honesty and a follow-up.