If you have accidentally deleted a session, the OKRs of this session are not lost and you can retrieve them. When you delete a session, its OKRs are stored in Objectives without sessions section.
To access the OKRs of a deleted session, perform the following:
In the navigation pane, under OKRs, click All sessions.
At the end of the lists of all sessions, click on Objectives from deleted sessions.
Your OKRs from deleted sessions will populate here, you will have to move the OKRs in this section to an active session in order to modify it's contents. You can do so by clicking on the OKR to open up the Objective page, then select the Move button to select a session to move to, or an OKR to align an OKR under within an active session.