Lead with a one-line summary
Say what is broken, for whom and how badly in the subject. Engineers triage from the subject line.
State the impact
Give the number of affected customers, the plan or account value and any deadline. Impact decides priority.
Give steps to reproduce
List the exact steps, the account or ID, the time and what you expected against what happened. Attach logs or screenshots.
Say what you already tried
List the checks and workarounds so nobody repeats them, and what the customer was told.
Ask a clear question and name the next update
Say what you need, such as a fix, a diagnosis or a workaround, and when you will update the customer.