Pr card has to be submitted during oath ceremony regardless it has expired or not.
Here is the link:
http://www.cic.gc.ca/english/citizenship/cit-ceremony.asp
I am not sure what needs to be done however I think report from police may be required. Contact CIC call centre and wait for someone in this forum with the similar experience to respond.
Losing a PR Card is serious, as it is a travel document and it has potential to be misused.
Call CIC at once to report that it is lost, so that they can update their system and to prevent misuse of card. File police report and perhaps submit a copy to IRCC right away.
Let the citizenship officials/office know about it ASAP