pc randomly turning off, but working good when it turns on

First of all which model of the Gigabyte B550 do you have? There are at least 12 of them?

Did you notice anything in particular that happened before the shutting down started? Windows or driver updates, browsing a dodgy web site, emails, etc.?

Well it is not overheating so that is good. It could be a hardware problem or a software problem

As there are many possible causes of your problem I recommend keeping a journal or a check list of what you try so you don't get lost. Believe me it is easy to forget what you have tried.

Hardware:

• Try going into the BIOS/UEFI and just leave it on for a while to see if it still randomly shuts down. If it does then it is likely a hardware problem.
• Faulty hardware - try isolating the problem by removing one component at a time, with the power shut off such as HDD/SSD/NVMe, Wifi card, graphics card (I believe all the B550 models have onboard graphics)

Windows:

1. Click on Windows Event Viewer which will enable you to view the date, time and user details of all shutdown events caused by a shut down (power off) or restart.
2. Use, with Administrator privilages, DISM command to repair Windows' images and SFC command to check for corrupt system files. Look up to learn how to use with the CMD prompt or use a tool like this that does it automatically for you (click here) .
3. Look in the Device Manager to sea if any device have the exclaimation mark indicating driver problems
4. Updating all drivers and Windows or rolling a recent one back.
5. A full viruse and malware scan would be a good idea.
6. Sometimes enabling "Fast Startup" has been known to cause random shutdowns. try Turning it off.

There are other possible causes and solutions such as intermittant power supply or mainboard problems. The above should be enough for now to keep you busy.