When the previous stage is blank for a closed Opportunity, then it cannot be reopened by the Owner as well. In order to reopen a Opportunity, previous stage is mandate which is not ideal. This creates unnecessary hassle on the user who might want to reopen a Opportunity to modify the data. The Alternate solution is to reach out to Sys Admin, find out the Opportunities, import the Previous stage via Data Import and then reopen the Opportunity, where a normal user has to go through all these steps just to reopen his own Opportunity.

Please vote for this idea so that a user should have the option to reopen his own Opportunity, whether the Previous stage is available or not.