Verified answer

A hard pedal is usually a bad brake booster or a problem with the booster. When you slid the vacuum hose may have come loose from the booster. I would check the vacuum first. It also may have a crack in it as well. In the cold and during a short slide there could have been just enough stress to crack the vacuum line or pull it loose.

(Unless you ran over something I doubt the Ebrake is causing the problem plus the propotioning valve would over ride an ebrake problem. Even if you have the ebrake on you still get a pedal plus you would notice it hard to drive if the ebrake was causing the problem.)

Ice could be built up on any E-Brake cables. Also could be some damage to the brake shoes or drums ( or disks ) you could also put a bit more info like make,model,year, mileage and what not. it could help if its a common problem.

good luck

might be something hanging on E brake cable look under it in back and then also brake booster is what makes power brakes work