How to Fix Old PC with Black Screen, POST Code 00h, and No Beeps After CMOS Battery Replacement and BIOS Flash?

I’m facing a serious issue with my old PC featuring an APC Raptor AT-11/04/03 motherboard and AMIBIOS from American Megatrends (1997). I was getting a “low CMOS battery” error, so I soldered a new lithium battery onto the Benchmarq bq3287AMT RTC module (reference video: YouTube link).

After powering on the PC, I encountered a black screen, continuous single beeps every second, and a POST code of 33h. According to the documentation, this means the BIOS couldn’t find the AMIBOOT.ROM file in the root directory of floppy drive

A. The keyboard also stopped working.

I attempted to flash the BIOS using a floppy disk with the latest version I found online. Unfortunately, things got worse—now there’s no POST, no beeps, just a black screen, and a POST code of 00h, which indicates: “Code copying to specific areas is done. Passing control to INT 19h boot loader next.”

I’ve already tried the following steps without success:

Replacing and reseating the 128MB RAM (cleaned with an eraser).
Resetting the CMOS using jumpers.
Using a different monitor.
Removing all external cards and peripherals.
Reflashing the BIOS.

I’d appreciate any advice on how to recover the system or troubleshoot further. Thanks in advance!