A note with few hundred lines of data displays red dots (in edit mode) instead of the actual content and completely blank in 'preview' mode. Cursor placed on any red dot displays "\u0"

Are you able to open that .md file with another app? If not, I’m afraid your note may have become irrecoverably corrupted.

I hope you have backups. If you don’t, do yourself a favour and try this plugin, a cloud backup solution or, at the very least, regular manual backups.