Moving from passive voice to active voice in a sentence

As various people have observed in comments posted beneath the original question above, the reason that the original sentence is so concise is that (1) it omits identifying who or what sent the business home, and (2) it uses "the business" as a short-form proxy for the identity of the people who were working from home.

You can supply the missing information in two steps, starting with the identity of the person or thing that sent the business home. Here is the original sentence:

During the COVID-19 lockdown, most of the business was sent to work from home for several months.

Here is a version of the same sentence with the sender identified as "the company":

During the COVID-19 lockdown, the company sent most of the business to work from home for several months.

And here it is with the sender identified as "management":

During the COVID-19 lockdown, management sent most of the business to work from home for several months.

Supposing that "the company" is an adequate identity for the sender, you can now consider how to rephrase "the business" as human beings. Here is one possibility:

During the COVID-19 lockdown, the company had most of its employees work from home for several months.

Unfortunately, if the main point of the original wording was to say that employees were doing most of the productive work of the company ("the business") from home, the revised wording has lost that point. To restore it, you would have to reframe what went on at the workers' homes. For example:

During the COVID-19 lockdown, the company's employees performed most of its business from home for several months.

Even at this point, despite having hammered out a coherent and seemingly complete restatement of the original sentence in active form, you might still need to make various additional or alternative tweaks to reflect the writer's actual meaning fully and accurately. That's par for the course when you're dealing with a passive construction that omits critical details of the intended meaning.

It is exceedingly common—and quite natural—for writers to inadvertently omit such details from their sentences. After all, they (usually) know what they mean even if they don't write it all down. The trouble is that readers, without further clarification, do not; instead, they have to depend on what the text actually says.