My bass guitar has had some terrible fret buzz lately. The buzz is coming from the fret itself, not the head or anything. I tried differing the amount of pressure on the left and right hands. It will buzz when I pluck, pick, slap, pop, and tap.
A drop or something may damage the instrument, meaning something slips or falls out of place and fret buzz develops.
As Dr Mayhem and Wheat Williams mention, heat/moisture content/new and different strings/slipping nuts/slipping saddles can all contribute to fret buzz.
(Wheat has a great answer on the effects of car temperature fluctuations on an instrument, if you are interested)
As a solution for the moment until you find a source of the problem, I suggest raising the action at the bridge. You may have to adjust intonation if you do this. If your neck has a bow, you may have to adjust that also, because if you raise action on a bowed neck, fretting at higher frets can become more difficult than usual.
Help on intonating/setting action on the electric guitar, but can be transferred to electric bass as well.
Hope this helps!
You need to look at whether your set up has changed. If you are using a different gauge of strings the neck curve will have changed and you will need to adjust your truss rod to bring the curve back to ideal.
Alternatively, frets wear down over time which can lower the height of the strings. This is usually more noticeable at a particular fret.
Another possibility is that the nut or bridge is too low- nuts can wear down, so you may need a new one.
We have a few questions on set up, truss rods etc.