Invoice approval issue

Dear All,

W e are having an issue whith invoice approval. The approver was on holidays and created vacation rules, delegating rights of approval to his assistant. Invoice was raised during this period and the delegated approver could not receive approval notification. When the right approver got back, the invoice was also not found in his instance. Some how this invoice is missing in the system. How can I proeceed to retrieve this invoice?

Thanks in advance for your inputs.

Vitalis