Sandisk Ultra usb 3.0 - not detected on my computer

[marlow]

I bought a SanDisk Ultra 256 GB 3.0 flash drive which worked the first time I used it (I put important files on it) but now it doesn’t appear on my PC (Dell XPS 8500 with Windows 10). It’s plugged directly into a rear 3.0 port. All 3.0 and 2.0 ports work. I haven’t had the opportunity to test it in another computer. I’ve looked in disk drives, DVD/CD - ROM drives and USB root controllers, all of which are working. I’ve also uninstalled/reinstalled the USB Root Hub (3.0) and updated the drives for Windows 10 from Microsoft. It’s set so that the computer won’t turn off the ports to save power. All of my other flash drives work properly. I don’t understand why it appeared and worked fine the first time but now it doesn’t. Please advise. Thanks.