Do you know how to run a My SQL query?
Maybe you have root/shell access to the server or you have PHP My Admin installed?
If so, you need to run this My SQL query on your XenForo database:
REPAIR TABLE xf_session
That's because the error is currently: Mysqli prepare error: Table './investpl_xenforo/xf_session' is marked as crashed and should be repaired
If you do not know how to do this, you may need to contact your host and tell them the error, and what needs to be done to fix it.