I have the same problem. My headphones (which aren't very nice or expensive, they're just kind of like Apple headphones) worked for a few months after I bought them, but a few days ago, I realized that the bass was completely mute, so that I could hear only the highest pitches. This has happened to me before, so I did some research on it. There are two significant variables to a headphone problem: the jack (plug) and the stereo wires.
When you look at your headphone jack, you should see that there are metal sections divided by white or black (may be other colors) plastic rings, with a metal section that is differently shaped from the others. Before the first section, there is a rubber jack grip (the part that you hold when you plug the headphones in). There are three basic types of headphones: standard stereo, iPhone, and iPod AV. Standard stereo headphones have only two metal sections along with the section at the end that is differently shaped. The section closest to the grip is known as the "ground". This is the part that picks up the bass current. The next section is the treble for the right earbud. The END section is ALWAYS the LEFT treble, no matter what kind of headphones. Beats are an example of standard stereo headphones. iPhone and iPod AV headphones have three metal sections, along with the end (left treble section). The only difference between these is that the iPhone headphones have a microphone section, while the iPod AV headphones have a video section, which is used mainly when the iPod is plugged into a TV or computer source. So, let's just leave out the iPod AVs. The iPhone headphones have: (in order from grip to end) a microphone section, then ground, then right treble, then left treble (end section). You most likely have standard stereo or iPhone (which includes any headphones that have a mic, and can be any company, not just Apple).
The second main sources of headphone problems are the stereo wires. These are located in the thicker rubber wire section that connects the jack to the headphones. The wires (if you have iPhone headphones) split midway, with the microphone wire stopping at the mic, of course, the left and right treble wires splitting apart and going to the respectable side or earbud, and the ground (bass) wire splitting with one half going to the right and the other to the left, creating an equal sound in each side. Headphone wires may have an
additional cord for strength, but this is irrelevant to the problem.
Now that I have thoroughly and boringly discussed the possible source to the problem in your headphones, I will now explain what the problem may be. If the problem is in the jack, since your bass won't work, it means that the ground section (1st closest to the grip if standard stereo, and 2nd closest if iPhone) isn't picking up the bass current correctly, or at all. This may be the device's fault. If the problem is in the wires of your headphones, it means that the ground wire is either severed or has an interference between the jack and the right/left division. An extremely strong sound current has the potential to cause the bass to stop working. There are ways to repair a broken ground wire, but I do not know enough to tell you how.
I hope this has been helpful to you, and I hope you can fix your headphones!