Completing a period end is a permanent action - this cannot be reversed. If you are using Dimensions version 2.51a and above, and have working periods enabled, you may be able to post into the relevant period if you have permissions.
The only way to reverse a period end is to restore your database to the backup taken prior to running period end.
Note : Be aware that by restoring the backup, you will lose all transactions and changes made since the point of the restore.
To restore your database, please speak with the person in charge of your SQL server. Alternatively, if you are an Access Hosted Client, please request this from support