They can go away and come back. Most heart murmurs
that present in young people are what are called innocent heart murmurs. They are not dangerous at all and are quite common. You should still be able to exercise, play sports, or do whatever you want.
It is very possible that it went away. It may come back, but the fact that it went away suggests that it is in fact an innocent heart murmur therefore even if it does come back you are in no danger.
When I found out about my heart murmur, I was scared at first because I didn't understand what it all meant. I thought I had some heart defect that would kill me before I was 30. But that is not true. Innocent heart murmurs are common and not anything to get scared about. You and I may have our heart murmurs come and go throughout our lives and still live to be 100. You'll be fine.