Why were the martians sleeping?

Question

In Doctor Who (2005) S10E09 Empress of Mars, we learned that martians were asleep for 5000 years.

But, why did they go into hibernation in the first place?

Answer 1

The tardis.wikia.com/wiki/Ice_Warrior page is well researched and has a section called "The Fall of the Ice warrior" from which the following is copied verbatim:

...a visit by the Fifth Doctor and his companion Amy led to the fall of the race. The two time travellers were looking for a decaying segment of the Key to Time, one of which was took the form of a pyramid's capstone. The segment had begun its decay, causing earthquakes. The Doctor and Amy, along with Zara and Harmonious 14 Zink, who were also looking for the segments, reached the top of the pyramid. Zara, who was a sentient tracer, activated the segment, leaving with Zink's time ring. This led to the formation of a gravity well which sent earthquakes and hurricanes across the planet for three decades. These devastated the Martian civilisation. Some managed to get off world but most remained on Mars where they fought amongst themselves for food and shelter. After thirty years, the energies from the segment of the Key to Time had been spent and the ground had settled. What remained of their race emerged onto their world where they planned to rebuild, even if it took them a thousand years. (AUDIO: The Judgement of Isskar)

Despite this honourable goal, the Ice Lord Izdaal had been observing the sky. His studies determined that their atmosphere was no longer capable of keeping out radiation from outer space. This would eventually kill them all. He told the government that their turmoil was not over and with the evidence of sickening children, he declared that their world was no longer sustainable. (AUDIO: The Judgement of Isskar) He was ignored by his people. Izdaal made the ultimate sacrifice. To prove himself right, he elected to step outside and face the Red Dawn, knowing he would die. (AUDIO: Red Dawn) His death led to his people coming to the same conclusion and they worked to survive, slowly becoming a conqueror race that took what they wanted from others. (AUDIO: The Judgement of Isskar) A group of Ice Warriors remained in suspended animation on Mars to protect Izdaal's tomb. Risking never being woken up, Izdaal's guardians lay inside their tombs waiting for a lifeform with enough honourable intentions to make it past the biometric locks inside the tomb, having "much" to offer to the Ice Warriors as they would to any Earth lifeforms that would discover the tomb. (AUDIO: Red Dawn)

According to the Twelfth Doctor, Mars' atmosphere had "all but evaporated", and the surface was "lifeless" before 1881. He told Iraxxa that the Ice Warriors could not survive without help. Friday agreed with the Doctor. (TV: Empress of Mars)

At some point prior to his travels to ancient Mars, the Fifth Doctor was surprised that there were Ice Warriors still around by the 21st century; he assumed they "all" left Mars after its atmosphere thinned "centuries" before the 21st century, while Lord Zzaal said the Ice Warriors had discovered "primitive, early life" developing on Earth before his people went into suspended animation. (AUDIO: Red Dawn) The Eighth Doctor dated this time as "many millions of years" before the 23rd century. (AUDIO: Deimos)

The Eleventh Doctor claimed that the Ice Warriors were scattered "all across the universe". (TV: Cold War) After the fall Edit

Following the destruction of Mars' atmosphere, the Ice Warriors fled to Deimos, one of the planet's moons, where they constructed catacombs and placed themselves in suspended animation in the hope of one day either reclaiming Mars or conquering Earth. A human-made exhibit on Deimos dramatising the lead up to Izdaal's walk into the dawn mentioned that Earth was unsuitable to be conquered and inhabited "at this point in history" and that the Ice Warriors had nowhere to evacuate to. It said that Ice Warriors would one day awaken from the tombs to become "rightful" rulers of the Sol system. (AUDIO: Deimos) Lord Zzaal, whom had placed himself in suspended animation around the same time as those on Deimos, also told the Fifth Doctor that at that point the Earth's environment was unsuitable for them. According to Lord Zzaal, even without the threat of the dawn, the low temperature and loss of water would mean an Ice Warrior could only survive for a few days. (AUDIO: Red Dawn)

As Mars died, one million Martians remained trapped in suspended animation on board the generation ship that had become the moon Phobos. (PROSE: Crimson Dawn) Similar installations existed in the asteroid belt in the Sol system. (AUDIO: Deimos)

Answer 2

Actually, they didn't intend to sleep for that long, as Friday said to Iraxxa after waking her up:

FRIDAY: By the moons, I honour thee, Iraxxa. My Empress, my Queen.

IRAXXA: My Sentinel. You have fulfilled your pledge. I am resurrected!

FRIDAY: Forgive me, Majesty, but we have slept longer than anticipated. By my calculations, Majesty, we have slept for five thousand years.

IRAXXA: Five thousand years?

Why did they go into hibernation in the first place? It wasn't explained in the episode, but it is generally accepted that the martians went into suspended animation when the planet's atmosphere was evaporated, as explained in Kerr Avon's answer.