Completing a period end is permanent and cannot be reversed. If you are using Dimensions version 2.51a or above with working periods enabled, you may be able to post into the relevant period if you have the correct permissions.
The only way to reverse a period end is to restore your database to a backup taken before the period end was run.
đ Note: Restoring a backup means you will lose all transactions and changes made since that point.
To restore your database, speak with the person responsible for your SQL server. If you are an Access Hosted customer, contact support to request a restore.