Have you checked how much free memory you have? Also there is a log file for media encoder you can bring up from the Media Encoder UI (not at a computer just now - I think you right clicked on the failed job).
Or are you not even getting to media encoder? Is it CH coming up with the error message?
Sometimes I have found exiting and restarting CH and Media Encoder cleared some issues for me. But screenshots or similar here might help - its hard to understand what you are doing exactly.