I normally don't like quoting from the #birdsite but it's unavoidable. Snapshots below taken from original tweet here:

https://twitter.com/flightradar24/status/1609399161228517379

Image on the left shows #AirTrafficPH from 2 weeks ago. Image on the right was during the height of the #NAIADown crisis. Full Senate investigation ito sigurado 😬

Piecing it together. It was no power outage - the UPS(Uninterruptible Power Supply) of the main CNS/ATM system failed ( probably due to a power spike it couldn't handle). Whatever they did to try to fix it, the backup CNS/ATM system (same site) also got hit & failed.

What a mess. It'll take 3 days to fix the air travel backlog - betting it's a week 🤞

Officials cite ‘outdated’ system for glitch that closed Philippine airspace | #NAIADown #AirTrafficPH| GMA News Online
https://www.gmanetwork.com/news/topstories/nation/856026/officials-cite-outdated-system-for-glitch-that-closed-philippine-airspace/story/

It looks like the main CNS/ATM system went down due to the reported power outage & then the backup CNS/ATM system also failed. Was the backup system ever tested?

"In a press briefing on Sunday night, CAAP Director General Manuel Tamayo said the air traffic management system was restored at 7:45 p.m. or 10 hours after the system went down at 9:49 a.m."

Technical glitch in NAIA air traffic system fixed; full operation restored | #AirTrafficPH #NAIADown | Manila Bulletin
https://mb.com.ph/2023/01/01/technical-glitch-on-naia-air-traffic-system-fixed-full-operation-restored-at-745-p-m/