Powering the router down without disconnecting the ADSL via the router's interface is usually picked up by the exchange as a dropped connection and if a burst of noise is picked up at the same time, as can happen, you'll end up on a slower profile than you were previously.
I'd definitely recommend leaving the router on all the time, and if you can try and get it to resync (disconnect via the router's interface, power down, then power back up again and allow to connect as normal) around midday today if possible. That should give you the best possible sync speed.
Hope that helps,
Plusnet Customer Support