Emergency

I have a iMac 21.5“ late 2013. i i upgraded the ram to 16 GB and also installed a 1 TB SSD on the motherboard. Im running the 2.0.1 Patcher with the latest Sonoma. All was working wonderful until yesterday evening when the screen froze and nothing worked anymore (no input response). I had to force shutdown the iMac. I wanted to restart but the iMac didnt boot. There is only a black screen for a long time until a blinking question mark folder icon pops up.

Please help me. I have no clue what it is and how to resolve. I tried to reset pram bit it didnt help