However, to my disappointment, the restaurant was closed.

disappointment 是由动词 disappoint（使失望）加上名词后缀 -ment 构成的名词，表示"失望"的状态或感受。dis- 是表示否定的前缀，appoint 原意是"任命、指定"，所以 disappoint 字面意思是"使...的期望落空"。