If this is a security/breach kind of problem, there are a number of factors that would be need to addressed.
1. Repair any damage done using backups
2. Determine what allowed the issue to occur in the first place
3 Completely convince all involved that your fix to #2 is truly solid
4. Deal with the PR end of things, as clearly some real explanation is needed after almost 4 days down
5. Alert any third parties about any lasting effects due to #1
Only after all these are done can the system come back up. There are likely other factors as well that only those on the inside may appreciate.
For most firms #1 is probably the easy one. Items #3 and #4 are harder.
1. Repair any damage done using backups
2. Determine what allowed the issue to occur in the first place
3 Completely convince all involved that your fix to #2 is truly solid
4. Deal with the PR end of things, as clearly some real explanation is needed after almost 4 days down
5. Alert any third parties about any lasting effects due to #1
Only after all these are done can the system come back up. There are likely other factors as well that only those on the inside may appreciate.
For most firms #1 is probably the easy one. Items #3 and #4 are harder.