I have the following passive clause:
The money was stolen.
Is there any transformation of it into active voice? Or it will remain like this?
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There are two types of passive clause:
The money was stolen.
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:
Passive: The money was stolen.
Active: *Stole the money.
The active clause is ungrammatical. It needs a subject, but it doesn't have one.
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.
If you make the transfer from passve to active or vice-versa, subject and object switch roles.
I (S) open the door (O). <-> The door (S) is opened by me (O).
In your example, there is only the subject "the money". In order to make the transformation, you need to "invent" an appropriate object, like "by someone" before you can switch to active:
Someone stole the money.
Now you have to decide for yourself wheter this sentence matches your specific needs or whether keeping the passive structure is the better choice.
One of the uses of the passive voice is to omit the subject, because it can be unknown or irrelevant.
If you want to convert the sentence into the active voice you should assume a subject, it can be a noun or a pronoun.
The money was stolen (by them).
They stole the money.
I supposed that the subject could be "they". But since the original sentence has no agent complement, you cannot convert exactly the sentence from passive voice to active voice, but you can always suppose a subject. A pronoun sounds to me more discreet, since it does not really reveal the subject identity.