Acknowledge the impact
Say the outage hurt their work before you discuss money.
Check what actually happened
Look at the incident record for start, end and which features were affected.
Apply your policy
Compare the downtime with your uptime promise or credit terms.
State the decision plainly
Say whether a credit applies, how much and when it will appear.
Offer something when no credit applies
If it is outside the policy, consider an apology note or a small goodwill gesture.