I am not sure what the specifics of your assignment are, but I can in a general way help you get started.
First, you might want to focus on what a "reflection" is. This is a subjective response to a work of literature or art: what does the piece in question say to you? You are not being asked for a right or wrong answer, but for your opinion. If you are asked to write about a poem, for example, you could jot down your emotional response: Did it make your heart sing? Did it confuse you? Did it depress you? You might write down a line you didn't understand. Or, did a certain line jump out at you because you could picture it in your mind's eye? You might note this too. Often, a response might be that a poem bored you—what you need to do is dig a little deeper as to why. It most likely bores you because it seems to have no relation to your life. Why is this? Is it because of the language? Or does it describe an experience outside of your experience?
Second, you might want to shift your point of view to that of your instructor. What do you think he or she wants? Often, in my experience as an instructor, I hoped for evidence in a journal that a student was actually engaging and struggling with a text. What was said was far less important than the idea a student was thinking about a work. Points of struggle and connection can be especially important to note, because that can guide your instructor's teaching to be more helpful. For example, when I was doing Jane Austen's Emma with a class, it helped when students journaled that they had no idea what a governess was—this showed me not to assume that students knew this, and I could spend class time filling in this gap.
Finally, if you are having writer's block, it can help (maybe starting somewhere separate from your journal at first for a few moments) to simply free write. Think about the topic you are supposed to be reflecting on and write what ever comes into your head without stopping, no matter how silly it seems. If you can't decide what to write, just write "I don't know what to write" until more thoughts come. This is a proven method that helps many people to unblock. Often we don't know what we are thinking until we start to write—and then it begins to flow out.