Does Zakat become obligatory on a sum invested in trade for three years that has not yielded profits currently and whose principal cannot be recovered, and on a sum invested in a real estate project that has been stalled for one year out of a total of four years?

As for money paid for investment, if the principal is guaranteed as a condition, the contract is invalid (fasid). This is because mudarabah (profit-sharing) is a partnership in which the capital provider (rabb al-mal) bears the loss alone, and the agent (amil) does not bear it unless due to negligence or transgression. In an invalid contract, the capital provider is entitled to all the money and profit, and bears the loss, while the mudarib (agent) is entitled to a fair wage (ajr al-mithl).

As for money paid to a contracting company, if it remains in its original state and reaches the nisab (minimum threshold for zakat), then zakat is due on it. If it has been used to purchase a building for demolition, then no zakat is due on it. Your share of the land's value is subject to zakat.