Q: imac mid 2011 black screen problem

Three beeps means there is a memory problem, so the system won't boot up. First try reseating the memory and see if that makes a difference. If not, try swapping cards, try running on one then the other. This Apple note tells you how to remove/install memory. If none of that works, try substituting known working memory. If that still doesn't work, take it into Apple for service.