Guitar string hitting fret while playing after changing to thing strings?

well depending how the truss rod (an adjustable pole in your neck) is set up in your guitar, it sounds like it needs to be loosened. but if you are not familiar with this then DON'T MESS WITH IT. too many people try adjusting these with no knowledge of it and end up really screwing it up. take it to a guitar shop to a proper luthier and have him adjust it.

If you're having a problem with fret buzz after switching to lighter strings, you need to adjust your truss rod. The truss rod is basically a big threaded rod with a nut on the end that counters the tension applied to the neck by the strings. When you switched to lighter strings, the truss rod was still applying the pressure needed to counteract the tension of the heavier strings, which is now more than necessary, causing a slight backbow in your neck.

I wouldn't recommend trying to fix this yourself. Take it to a local guitar shop and they can fix it for you. They will probably be willing to show you how to fix it yourself as well, as it is a fairly rudimentary thing.

Need to know if it's electric or acoustic as the answers will differ. Acoustic: the braces may have been knocked loose. The neck may have been damaged and loosened from the heel joint. Electrics inside might be loose. Electric: Check the neck, check the frets where you notice buzzing, Check the machines (tuners) to see if they're loose. etc Best bet would be to take it into a music repair shop and have them check it out.