Drive LED flashes once then nothing happens

I have a PC with Gigabyte B550M DS3H AC motherboard running Windows 11, version 24H2, with Samsung SSD 980 1TB. It suddenly wouldn't boot up today. Symptom is, from power up, one flash of drive LED then a black screen with a cursor in top legt corner. The boot sequence doesn't proceed from there. After 4 power up cycles, it booted normally.

What should I check first of all, please? I've run chkdsk on c: and no problems were found.