Are you able to see if there is any js/console errors? (I’m new myself) perhaps view on another computer on the network?
A New Chapter for MagicMirror: The Community Takes the Lead
Read the statement by Michael Teeuw here.
Read the statement by Michael Teeuw here.

Posts
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RE: Blank Screen on Pi 1 after successful Install
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RE: Complete Setup tutorial: npm command not found / unable to install dependencies
I had same problem so I ran this:
curl -L https://npmjs.org/install.sh | sudo sh