Dead Low E string on my Classic 60's strat

My lake placid blue classic 60's strat has been my number one guitar for years. I fitted it out with Kinman Traditional Mk II's and it played well, sounded great and felt super.

However, a few months ago I started having problems with notes at the 15th fret choking out really badly on the low A and E strings. I don't remember anything, but I presumed I had knocked the guitar against something and knocked a fret out of true or something. So the guitar went off to the repair man, and came back playing fine once again. However, the bottom E string just sounds dead now. Really dull compared to the other strings. It's mainly noticable on the lowest notes from about A on down, although you can hear that the string is dull sounding anywhere on the neck.

I've wondered it I'm imagaining it, but the change is quite marked. The guitar used to be really resonant and tight sounding on the low strings, but every time I hear it now I wince.

Has anyone else experienced something like this? How did you solve the problem?

 

Thanks guys. I checked the screws holding the neck on and they weren't quite tight. After tightening them up, everything is absolutely back to normal. Phew.