Emergency changes are specific to the organization and need to be defined on case-by-case basis. Meaning, they depend on the organization, services, processes in place, roles and responsibilities, etc. Therefore, it's not realistic to have common procedure for all possible cases.
However, what we have in our toolkit is definition of Emergency Change authority and respective roles (e.g. ECAB and its members).
Also, following article can provide you more information „How to manage Emergency Changes as part of ITIL Change Management“ https://advisera.com/20000academy/blog/2016/01/19/how-to-manage-emergency-changes-as-part-of-itil-change-management/