@MichaelKim - We’re going to need more information than that. So answer some questions, if you please:
Was this a new installation or upgrading from a previous version of Magic Mirror?
If you point a web browser at your Pi’s network address, does it show anything or just a black page?
Is there anything in the pm2 logs that says “error”?
Have you tried stopping the Magic Mirror process and manually running “npm start dev”?