The European Space Agency (ESA) will attempt to reestablish contact with the Beagle-2 at the next planned communication session, set for June 27, said Rosaviakosmos, quoted by the Ria-Novosti news agency.
It did not indicate when the initial communication failure occurred.
Ria-Novosti said the problem lies with the ESA's orbiter.
Mars Express was launched on June 2 and is expected to reach Mars in December. Once there, it is to drop the British-designed Beagle-2 lander, which will collect and analyse soil samples in the hope of finding traces of water.