There are two problems a missing tooth can generally cause. One, as you are finding out, the tooth
above will supra-erupt. Eventually that tooth
will be lost if it's already dropping and continues, unless you get some opposition to it.
And, second, the tooth behind will try to drift forward to close the gap. This can also cause loss of that tooth.
Please go and be evaluated for either of those two conditions and see what is your best course of treatment.