I can't see my flash drive in finder

Use Disk Utility to see the drive's format and other status. If you no longer plan to use it on a PC then reformat it for a Mac.

Drive Partition and Format - El Capitan or Later

1. Open Disk Utility in the Utilities' folder.
2. After Disk Utility loads select the flash drive (out-dented entry with the mfgr.'s ID and size) from the side list.
3. Click on the Erase tab in the Disk Utility toolbar. A panel should drop down.
4. In the drop down panel set the partition scheme to GUID. Set the Format type to APFS (SSDs only) or Mac OS Extended (Journaled.)
5. Click on the Apply button and click on the Done button when it is activated.
6. Quit Disk Utility.

What happens if the flash drive does not appear in Disk Utility?

The flash drive was working a few minutes ago and then... nothing. I tried looking for it in Disk Utility, but it does not appear in the sidebar. That said, I cannot even erase and reformat the flash drive. Is the flash drive dead and unusable or is there another way to get at it.

I have tried it on multiple Macs running OSX High Sierra. While getting the data back would be preferred, restoring the flash drive is acceptable.

Please advise.

Thanks.

Have you tried Disk Utility?