Try to simplify and extract the four or five main points of the lecture, and write them down for future revision. It is difficult to recall things if you try too hard. Some times if you go on with the rest of an essay, the missing fact will come to your mind. It is also useful to have other lines or links to the information. Think of you memory as a three dimensional grid with alternative pathways.
Mnemonics are often useful.
For example I could not recall the name of the wild geranium that you can leave in the garden over winter. At that time the University was doing quite a lot of building, and building cranes were sticking up over the horizon. This association of places or ideas is a classical method of remembering used by the Romans and Greeks. There are many books that describe how you can link and organise your memory, both for giving talks and for a longer term memory. If you have not done any work or revision then you will spend so much time looking things up that you will not finish the exam.