I took a test in class and found it to be difficult. So I asked in my WhatsApp group if others felt the same. 4/36 people replied yes they felt it difficult. Others did not respond either in yes or no. They may reply later. Can I infer anything about the whole class from this response. How to start a statistical approach to analyzing this situation about how the class felt?
No, you can't infer anything about the whole class from the people who responded because the people who responded are almost certainly not a random sample from your class. They are probably a biased sample, although the direction of the bias may depend on exactly how you worded the question.
For instance if you ask:
I found the recent test to be difficult. Did you? (YES/NO)
you may get different answers from a question like:
Did you find the recent test difficult? (YES/NO)
How did you find the recent test (EASY/DIFFICULT)