my three way lamp is suddenly just one way. is this an electrical problem that could be a fire hazard? Why would a lamp just suddenly stop being three way after being plugged and unplugged?
The most likely explanation is that one of the two filaments inside 3 way bulb burnt out or broke.
If it was already close to burning out before you moved the lamp, the mechanical shock of moving may have broken it the rest of the way -- perhaps it broke completely so you didn't see the characteristic "flash" of a filament burning out when you turned on the lamp for the first time after moving it.
You could test the lamp contacts in the socket with a continuity tester at each switch position to verify that this is the case, but a much easier (and definitive) test would be to replace the bulb with a new 3-way bulb and see if it works.
I had this same problem. First I tried a "new" bulb & it did the same thing, so I Johnreplaced the socket, and that didn't fix the problem. I tried the "new" bulb in another lamp and confirmed that it was defective. The 3rd "new" bulb was good and corrected the problem. The bulbs were labeled GE, but "made in China." The lesson I learned is that sockets seldom go bad, as this is the 2nd time I have needlessly replaced one. Hopefully I will remember this before I make this mistake again.