If you are using a Mac and the YubiKey is not showing any LED activity, that indicates it may not be receiving power. The SMC on Intel-based Macs controls the power delivery, and sometimes resetting it can resolve this issue. For instructions on how to reset the SMC, please see Apple's KB article below.
How to reset the SMC on your Mac
If you're using an Apple silicon Mac or an iMac, guidance is also provided on the page linked above.