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This is really a wide open question that is dependant totally on your network environment.

Here are some very general things that occur in most networks of any size:

Are all your external connections still alive?
Are all critical servers up and running?
Did any processes that run overnight (like backups) complete successfully? Do you need to change backup media?
What errors are showing in the Event logs?
Are there any oustanding trouble tickets from users?

Beyond this you need to come up with your own checklist based on your environment.