HP Laptop won't start, caps lock blinking 3 long two short

In this video we try to fix a HP notebook 15s series. The screen is blank, no boot, caps lock is blinking.
Some materials in this video:
- Some type of brush - for cleaning
- DeoxIT red - cleaning conductive surfaces
- 99% Isopropyl alcohol
I don't know what else... If I missed something, just ask, and I will update.
@southlondonrepair We are based in Addiscombe, South London. We repair consoles, Audio - Video equipment, televisions, vintage electronics and anything else what the customers can bring it in.
Customer come to us, as we can do same day repair - when is possible.
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