If PMI cannot reach your project contact, they might let you substitute a different project contact who can verify your experience or they might reject your application. If you cannot reach your contact, it could be because your contact does not agree with your application for whatever reason (including simple misunderstanding). If this is the case and they tell PMI that they do not agree, then that creates a big problem in getting approved. Therefore, if you can't reach your contact and get them to agree with what you have for their project, then you should not submit your application.
Your options for what to do if your contact does not respond include:
1) Wait until you can reach the contact (by phone or in-person if possible).
2) Use a different contact who can verify your experience for that project (a different primary stakeholder, a sponsor, a client, or a manager).
3) Substitute a different project that was the same number of months (or more).
4) Wait until you have 36 months of experience not including that project.
5) Not recommended: Submit your application with the current project contact listed and hope they don't try to reach your contact and that your application is not randomly selected for audit.