Today I had to deal with this MM reopen period issue.
Problem: One user opened by mistake period 06 year 2005 but they still had documents for posting in period 04.
Solution: The solution is running the MMPI transaction and read note 48781.
This is a post I found on sap.ittoolbox.com SAP forum which explain some basic steps to reopen the MM period.
1. Lock all users out of PRD
2. Take snapshot of MB5L for all materials at company code for current and previous periods
3. Add User parameter MMPI_READ_NOTE with value = current date in this format YYYYMMDD
4. Stop all processing that updates inventory values
5. Run t-code MMPI with correct period
6. Run Z_DEl_HIST_ENTRIES program
7. Take MB5L snapshot for all materials at company code fs01 for current and previous periods
8. Validate that values match on MB5L pre-post snapshots or are appropriate.
9. Unlock all users in PRD
10. Remove User parameter MMPI_READ_NOTE from user ID
Good luck if you have such a problem.
Be carefully if you want to reopen a period that is far in the past!!!!