We all have bad days. Sounds like you had a big week; sometimes it requires more than two days off to recover fully from those. Your body isn't stupid. It isn't going to use muscle for fuel if there's fat to burn. And everyone's got some fat to burn. Nobody loses 80W off their threshold in a day.
Give yourself a break (cut yourself some slack and stop panicking), a day off, and a retest (in that order). You'll be good as new and firing on all cylinders.
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Cat 2 Track, Cat 3 Road.
"If you’re new enough [to racing] that you would ask such question, then i would hazard a guess that if you just made up a workout that sounded hard to do, and did it, you’d probably get faster." --
the tiniest sprinter