css hover not working on div

1/*there can be instances where the class or the id was not set well
2
3Make sure that you have your css link well set, e.g; */
4/*.class  if you used class="class" */
5/*#class  if you use id="class"*/
6
7.class{
8  background-color: blue;
9}
10.class:hover{
11  background-color: red;
12}
13
14/* OR */
15 #class{
16  background-color: blue;
17}
18 #class:hover{
19  background-color: red;
20}
21