@geckospotnixie - This isn’t really a lock up. The command your giving will run the MagicMirror process. It’s still running, spitting out messages to the console as things happen. You’ll get your prompt back when the process is complete. Since the process runs in a loop until something outside kills it (or if it crashes hard), you’ll never get your prompt back.
If you want your command line prompt back, type
ctrl-c and it will stop the mirror process and let you enter more commands. If you want to run the mirror process in the background from the command line, add the ampersand to the end of the line. Like so:
DISPLAY=:0 npm start &