Try to recapture the mood of the dream, which is important. If you still can't remember, but an event later in the day triggers your memory, write it down. It could be helpful in working out the meaning of the dream. Train yourself to do this regularly, and gradually memories of your dreams should follow. Some dreams, however, can be simply scary, and a particularly frightening dream is called a nightmare.
True nightmares occur in the non- dream phase of sleep, and have little remembered content.
I vividly remember a nightmare of my own. I woke up in a terrible state. In my dream I was walking around carrying my own dead body in a big wicker basket. What I had really seen in my dream was the casting off of my old self, getting ready to go on to new things. Death isn't always bad in a dream.