I hate it when that happens. I occasionally lose most of my power, but I can usually get it back pretty quick. A few suggestions.

1. Take a 1 week break.
2. You said you did a lot of drills already. Did this include the constant-grip-pressure 'awesome drill'? If that one doesn't work, then definitely take a break!
3. Have a practice session or two just swinging a club or other long-ish object (without hitting any balls). The exercise is to get a good swoosh-sound (back) in your swing with minimal effort.