Although the prefix “dis” in “disappoint,” as usual in English, means “not,” the story of “disappoint” is more than just a simple negation of “appoint” in its various senses.

Does disappointed have a suffix?

Only the noun retains evidence of its French origins, the suffix -ment in disappointment.

Is disappointed a root word?

The original French root of disappointed is desappointer, which has one meaning that's basically "to fail to keep an appointment." So you might imagine the word disappointed like this: you made an appointment with a friend, but he blew it off.