You should be hoping for the best but planning for the worst. Spend some time coming up with emergency/nightmare scenarios and build up a document with contingency plans. This document should also include emergency contact numbers and other vital information. Some examples of these scenarios include:
A fire starts in your venue.
Too much power gets used and a fuse in the building gets tripped.
A hacker has a medical emergency.
Someone is caught trying to steal something expensive like networking equipment.
The list goes on, and you should have instructions on how each one should be handled. Make sure all volunteers have access to the document and having a physical copy of it on-site can be helpful.
In terms of emergency contacts, you should include campus security, the local non-emergency police number (not 911) for situations that are not time-critical, and all of your main organizers, along with the times they will be available on-site.
You should also notify local emergency services that you will be hosting a large event with a lot of students, not from your school. They should have some protocol for large events and will ideally have a person on-site or on-call through the duration of the event.
It is impossible to prepare for every possible scenario, so if something unexpected happens, remember to stay calm, try to isolate the issue, and resolve it with a level head. Make sure at least one member of your organizing team is awake and alert at all times.