There are two types of passive clause:
Short passive
The money was stolen.
Long passive
The money was stolen by a short man wearing six jackets.
Most passives are short. They don't contain by-phrases.
Only long passives have corresponding active clauses:
Short passive
Passive: The money was stolen.
Active: *Stole the money.The active clause is ungrammatical. It needs a subject, but it doesn't have one.
Long passive
Passive: The money was stolen by a short man wearing six jackets.
Active: A short man wearing six jackets stole the money.This active clause is fine. It needs a subject, and it has one.
Your example is a short passive, so it has no direct active clause equivalent. If you want to make it into an active clause, you'll need to add a by-phrase, turning it into a long passive.
You'll have to rely on context to figure out what by-phrase works in your clause, if any.
In this answer, the * symbol marks a sentence as ungrammatical. For more information about passives, see A Student's Introduction to English Grammar (2005), chapter 15.