If you didnt delete the file itself, at the top of the code, there is a button to revert the version of the file, just click older version. Just put it back to when you didnt delete it.
If you did delete the liquid, then what you can do is reinstall the theme again as an unpublished theme. Find the footer.liquid and then create the footer.liquid on your main theme and transfer the code. Obviously this choice you will lose all your customization you've made on footer.liquid.
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