The government had been easing restrictions imposed two months ago to try to prevent the spread of Covid-19.
Shops, restaurants and hair salons had been allowed to re-open. But they now have to close again.
The efforts to fight the coronavirus have had a crushing impact on the economy, which was already in a dire state before the pandemic.
Lebanon started negotiations with the International Monetary Fund (IMF) on Wednesday to secure financial assistance it urgently needs.
Lebanon has reported 870 confirmed cases of Covid-19 and 26 associated deaths.
Last week, the government responded to a drop in the number of new infections by easing restrictions on businesses and places of worship in place since mid-March.
But Prime Minister Hassan Diab warned on Tuesday that more than 100 new infections had been recorded in the previous four days, accusing some of "negligence and lack of responsibility" for ignoring guidelines on social distancing.
He announced that a "total lockdown" - excluding the health, agriculture, food and manufacturing industries - would be imposed from 19:00 local time (16:00 GMT) on Wednesday until 05:00 on Monday in response.