Car seems dead - no auto lock functioning, no lights on dashboard etc

I last drove my Camry 2008 4 days back. Today tried opening the car with auto lock and it did not open. Thought that its a key fob battery issue, so opened the car manually and then tried starting the car.
No dashboard lights went on, no horn, basically nothing - seems as if car is dead. No clue what is the issue.
Tried with another pair of keys just to be sure and same issue persists.
Battery is 2.3 years old.
Any way I can do a self diagnosis and not spend $'00 on diagnosis in these challenging times. Any guidance appreciated.
Don't know much about cars so some handholding will be appreciated.
Thx