When the strings suddenly begin touching the lower frets on your guitar, the instrument becomes unplayable. This problem can originate at several different places on the guitar. In order to properly diagnose the problem, you need to know exactly what you're looking for -- a fairly simple task and, in general, a fairly easy problem to solve.
The truss rod, a metal bar inside the neck of your guitar, counteracts the force of the strings. If the truss rod is not properly adjusted for the gauge of strings you use, it slowly warps the neck -- a leading cause of strings suddenly touching the lower frets. Rest your guitar on a flat surface and look at the neck. If the truss rod is the problem, you should be able to see the neck bowing backwards. Tension can be adjusted by using a tool to turn the truss rod at the top or bottom of the neck.
Cheap plastic nuts have a tendency to wear away and even break apart. If this happens, some of the strings may end up resting against the lower guitar frets. Closely examine the nut, which holds the strings at the top of the neck, and look for any obvious wear or cracks -- problems with the nut are usually easily visible. If the nut is the problem, replace it with one made of higher-quality plastic, ivory, graphite or bone.
The bridge, where the strings attach to the body of the guitar, is another area that can create problems with string height. Try raising the height of the bridge in order to fix this problem. For guitar bridges with adjustable string saddles, like on most Fender Stratocasters, the height of each saddle can be altered with an Allen wrench. On guitars with a single adjustable bridge piece, like most Gibson Les Pauls, the height of each side of the bridge can be adjusted by turning a small screwlike piece. Owners of some guitars may need to visit a qualified technician.
Excessive humidity and dampness can cause guitar parts to warp, which can make the strings suddenly begin touching the lower frets. Depending on which part of the guitar has warped the most, there may be very little you can do to fix the problem. If your guitar is noticeably warped, take it to a qualified repair person and see if they can help you fix your instrument.