Hi Pubudini,
just remind
You must be an owner or reviewer in both the current step (active step) and the previous step to be able to reopen that step. If you are the Business Process Flow (BPF) instance owner you can reopen a step, regardless of owner or reviewer assignment, from the BPF monitor.
For the more details kindly go through the below link..
Criteria for Reopening Steps - SAP BusinessObjects Planning and Consolidation - SAP Library
Shrikant