The return from February vacation is the worst. No fun holidays to look forward to, no three day weekends, just 8 more weeks of straight out school. Mornings are cold, homework is boring, their friends annoy them, you annoy them, life just sucks.
It's no fun for you either. Nothing fun for you to look forward to. My suggestion, find something. Let your teen know that this is a tough time of year, and you "get" that motivation is at kind of a low point. Ask them if there is something they want to plan a month down the line that would give them something to look forward to. And help them plan. And by the way, do the same thing for yourself!
At the least, have reasonable expectations. It will take a week or two to get back in the groove. Understanding will get you way further than yelling!