- the bridge has indeed moved (worst case scenario) - the strings or some of them are old (easily fixed) - there is a compensation problem at the saddle or nut (easily fixed if you know how) - the action is too high (possible need to lower the nut or saddle or a neck reset in a worst case scenario) - incorrect fret placement (unlikely if it was OK before)
I would start by checking the easy and obvious things first like action and age of strings. What sometimes happens is that, with wear, strings can wear the grooves in the nut and saddle slots causing slight intonation problems. I have fixed many of these but you need a very fine needle file. Any decent guitar tech could help with that.