There are a number of reasons why a goldfish might lie at the bottom of the tank. Regardless of the reason, it's a good idea to get it up off the bottom by putting it in one of those floating baskets or containers that hang from the side of your tank. Wounds and skin irritations can develop from being on the gravel all the time.
Sometimes one fish is being chased by another. That fish will then stay at the bottom to avoid them. A female might get chased so much that she'll stay low to avoid any males. With either of these problems, it sometimes helps to keep the chased fish in the separate area described above for awhile.
It's also very possible that there is a water quality issue, so yes, a partial water change would be good. It may seem logical that if that's the problem, more than one fish would be staying at the bottom, but some fish are more sensitive than others. If you don't already have testing supplies to check your water for various toxins, a test kit would be a good thing to invest in. If you find a problem, continue the partial water changes until the water is good. It may also help your fish to add aquarium salt (never use table salt - it has anti-caking agents in it). Farther down, I'll give you a link to a site that explains how to use salt.
There are other reasons a fish will stay on the bottom. Some kinds of parasites can cause this. Sometimes a fish that is just stressed will lie on the bottom. You could add a water conditioner such as Stress Coat to the tank.
Here's a link that explains "bottom-sitting" and tells you how to use salt:
This link will take you to a site that has lots of good goldfish information:
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