Simple question. How do you run the Sprint Review and Sprint planning?
This is something that the project's risk management plan should address. If you haven't identified individuals at either the project or iteration level that are crucial to success and methods to continue work when they are unavailable, this is the perfect time to do so, as soon as the team is around. One of the things that you should do is identify alternates for roles so that no position is one-deep.
In the meantime, the entire team should be able to carry on, for the most part.
The Scrum Master can facilitate your Sprint Review. If you can have any customer or user representative available, that would be beneficial as you demonstrate your features to the stakeholders and receive feedback. Since the Product Owner is not present, it could be a good idea to make a recording of the meeting somehow - a video recording, audio recording, screen capture, and notes - and present them for review upon their return.
If your Product Owner is involved in your Sprint Retrospective, I would also continue that as planned, only without the Product Owner. You are, after all, reflecting on your team's performance and how you handled problems and met successes. The Scrum Master, as the keeper of the process, should be aware of the goals in terms of story points, features, and problems that arose. Again, since a critical stakeholder isn't present, I'd recommend capturing some kind of outline of the discussion and findings.
If your Sprint Review and Sprint Retrospective are one meeting, anyone who is not involved in the process directly should not participate in the retrospective. For example, I mentioned that a different representative of the customer or user group might be a participant in the Sprint Review instead of the Product Owner. If this person has not worked with the team closely, they would be asked to be a silent observer of the retrospective (which may be useful for training them as an alternate Product Owner) or leave the room entirely.
Upon the Product Owner's return, I would recommend that the Scrum Master and perhaps one Development Team member brief them on the results of the sprint review and sprint retrospective. With adequate recordings and meeting notes, the Product Owner should be able to read those and have a short discussion with the Scrum Master and team member if there are any questions or concerns.
As far as sprint planning, the Product Owner's job is to continually update the product backlog with new stories and maintain the priority. At any given moment in time, the product backlog should be prioritized. Your Scrum Master should be able to take historical project data and lead the team in the estimation process of stories. Then, the team can pull down the appropriate number of stories based on previous sprints.
It is obvious that when PO is not available nothing will be same. But of course team should make an effort to decrease the damage to sprint. I think SM should lead the team such that team can work on clarified tasks and leave the unclear tasks to time PO is available. In scrum way, the project continues step by step by leaving some parts to be decided later. So maximum availability is critical.