If you have accidently closed a period and opened one that shouldn't be open, then you need to check that no postings have been done in the new period.
If you have accidently closed down the last period and this was your year-end, then the period can't be changed back by Support this would have to go to the consultants as paid work.
For both of the above if you have checked that the period hasn't had any postings and the year-end wasn't the month you closed, please raise a new case online and reference the title of this article.