For some time, I've been unable to use my Mac to access some share drives at my place of employment.
So, whenever I needed to transfer files, I had to sneakernet them (albeit with a USB flash drive -- no walking involved).
I just had a big DUH moment.
I realized that I'd never updated the network passwords in Apple's somewhat easy-to-forget-about Keychain Access Utility.
I feel pretty dumb now.