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 😬

Flightradar24 on Twitter

“Here is traffic in the Manila FIR now and from 2 weeks ago at the same time. (Last Sunday was Christmas, so volumes lower anyway)”

Twitter

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Officials cite ‘outdated’ system for glitch that closed Philippine airspace | #NAIADown #AirTrafficPH| GMA News Online
https://www.gmanetwork.com/news/topstories/nation/856026/officials-cite-outdated-system-for-glitch-that-closed-philippine-airspace/story/

Officials cite ‘outdated’ system for glitch that closed Philippine airspace

Flight operations in Manila are expected to fully recover in three days, but transport officials said billions of pesos' worth of upgrades are needed to prevent a repeat of the technical issues that effectively closed Philippine airspace to hundreds of flights on Sunday, affecting thousands of passengers.

GMA News Online

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/

Technical glitch in NAIA air traffic system fixed; full operation restored at 7:45 p.m. Sunday

The Nation's Leading Newspaper

Manila Bulletin