Short Answer
Atman never loses its self awareness. You have the wrong idea. There's no material body in first place, then how can Atman acquire body consciousness?
न निरोधो न चोत्पत्तिर्न बद्धो न च साधकः ।
न मुमुक्षुर्न वै मुक्त इत्येषा परमार्थता ॥ ३२ ॥
There is no dissolution, no birth, none in bondage, none aspiring for wisdom, no seeker of liberation and none liberated. This is the absolute truth.
-Mandukya Karika, verse 2.32
Detailed Analysis
I know Atman is self aware but I don't know why that Atman lost his self awareness
Atman never ever ever ever loses its self awareness, no matter what's the situation. If there's one thing you should take away from Vedanta, it should be this one. Its the basic principle.
अमात्रश्चतुर्थोऽव्यवहार्यः प्रपञ्चोपशमः शिवोऽद्वैत एवमोङ्कार आत्मैव संविशत्यात्मनाऽऽत्मानं य एवं वेद ॥ १२ ॥
Mandukya Upanishad - 12
That which has no parts (soundless), incomprehensible (with the aid of the senses), the cessation of all phenomena, all bliss and non-dual Aum, is the fourth and verily the same as the Ātman. He who knows this merges his self in the Self.
This view that Atman gets overcome by ignorance and starts to see this world is utterly wrong. Your question itself is wrong, how can you expect an answer?
You are probably misled by the notion since it is said that "I am Atman", and since I see this world, it must be the case that Atman is in illusion. This is not the case.
When scriptures say "I am Atman", or "This world is Atman", or "You are Atman", it means that substratum of me, you and this world is Atman only like a Gold bracelet, Gold ring is nothing but Gold. To conclude that Gold has forms like bracelet, ring etc. is a wrong conclusion. Similarly concluding Atman gets overcome by ignorance is not correct.
एष एव जगद्.रूपम् जगद्.रूपम् तु न_ईश्वरे ।
हेम_एव कटक.आदित्वम् कटकत्वम्न हेमनि ॥३।६१।५॥
YV-3.61.5
World is Ishwar only but there is no world in Ishwar like Gold bracelet is Gold only but there's no bracelet in Gold.
