Make sure you have chosen the correct layer and the eye for that layer is on
We would need details, but usually the common cause would be selecting on the wrong layers. An adjustment layer for example.
Other than that since we can't directly look at your computer:
The more information you supply about your situation, the better equipped other community members will be to answer. Consider including the following in your question:
Adobe product and version number
Operating system and version number
The full text of any error message(s)
What you were doing when the problem occurred
Screenshots of the problem
Computer hardware, such as CPU; GPU; amount of RAM; etc.