In this work we theoretically study the structural transformations occurring in monolayer hexagonal boron nitride (h-BN) subjected to strong deformations. The first-principle computer simulations are based on the density functional theory employing the pseudopotentials with the exchange-correlation energy given in Perdew-Burke-Ernzerhof form. Depending on the direction and the type of the strain, the simulations reveal different types of structural transformations in two-dimensional (2D) h-BN associated with the elastic failure. It is found that, under certain shear strains, there can form an ordered 2D orthorhombic phase with the 2D layer space symmetry pmmm. Thermal stability of the latter up to ~900 K is demonstrated by first-principle Born-Oppenheimer molecular dynamics. The examination of electronic structure indicates that, in contrast to the conventional h-BN, which is an insulator with the wide band gap, the predicted orthorhombic phase is expected to be a semiconductor with the narrow band gap of ~ 0.65 eV.