You cannot undo the Close Current Period task. However, General Ledger allows you to post to and report on prior periods:

• Refer to the linked article below: "How do I open a closed period in General Ledger?" for information about reopening a prior period.
• Refer to the linked article below:  "How do I print financial statements or a year-to-date ledger for prior periods after the first period of the prior fiscal year?" for information about printing financial statements for prior periods.

Otherwise, restore your entire data folder from the backup that you made prior to using the Close Current Period task. This will set your data to a state at which that period was still open. Any work done since then will need to be re-entered. Refer to the linked article below: "How do I restore my data files?" for information about restoring your data from a backup.

DocLink: How do I restore my data files?
DocLink: How do I print financial statements or a year-to-date ledger for prior periods after the first period of the prior fiscal year?
DocLink: How do I open a closed period in General Ledger?