UPSB(iMac18,3): thunderbolt power on failed 0xffffffff\n

Welcome to Apple Support Communities.

I believe we need to proceed with an EtreCheck report. EtreCheck is a useful App to diagnose and identify issues, both hardware and software. It is trusted by many users. Please follow the steps outlined below:

1. Navigate to: https://etrecheck.com and get the free version of EtreCheckPro.
2. Download it. It is a free and trusted app. It is often recommended here on Apple Support Communities. 
3. Open EtreCheck, and before running the report, be sure to check “Allow full Drive Access” at the bottom of the EtreCheckPro window.
4. Run the report, and put it in a post, as seen here: https://discussions.apple.com/docs/DOC-250000211. We will then examine the report for issues in your Mac.

Cheers,

Jack

Your report indicates you may be having a hardware failure. The #1 priority is there is no backup for your computer which puts all of your data, settings, apps etc at risk. You should NEVER EVER operate any computer without a sound backup plan. Apple included Time Machine with every Mac, it only requires that the user have a dedicated external hard drive to backup to. Many experienced users recommend the OWC Mercury Elite Pro. Once you have your external HD then follow the instructions in Back up your Mac with Time Machine. I recommend you do nothing with the computer until you have backed it up!!!!!!!!!!!!!!!!

After you have backed up the computer make an appointment at your local Apple Authorized Service Provider to have the computer professionally diagnosed. If the internal HD is failing as I think it may be then post back and we can offer suggestions on how to use an external SSD which is less expensive than replace the internal HD and will improve the machines performance.