When you turn on a tap that should be working and nothing comes out, test other fixtures to infer where the frozen blockage could be. Finding out the location and extent of the freezing is the vital starting point, but there’s something even more important. When water freezes, it expands, often strongly enough to burst pipes. Always remember that a damaged pipe probably won’t leak water while it’s still frozen, but it could leak lots of water as it thaws. Always look for longitudinal splits as you’re defrosting pipes, and never leave your cottage with frozen pipes and the water still turned on.
Applying heat to frozen pipes is the goal, and you’ve got options. Pouring hot water on them is effective when you can get to the trouble spot and if pooled water doesn’t matter where you’re working. If the pipe is sloped, even a bit, hot water will run along the pipe and deliver thawing action into areas out of reach. A hair dryer works too, or you can bust out the heat gun (like a hair dryer, but super-charged), set on medium. Never use a hand-held torch or any open flame to thaw pipes. It’s a fire hazard and will damage plastic.
Pipes freeze most often in areas with little air circulation. Your cottage might be above freezing in the middle of the kitchen during a winter visit, but pipes travelling through a lower kitchen cupboard or behind shelves could still get cold enough to freeze. A room fan is surprisingly effective at defrosting pipes in an otherwise warm room. Pull back the furniture, open the cupboard, then train the airflow in the area of the frozen pipe. Keep a close eye out for water leaks until the pipes thaw and prove themselves sound.
Don’t be surprised if hot water supply lines freeze before cold ones (it’s called the Mpemba effect and it’s worth a google). And leave freeze-prone fixtures running a small steady stream to keep the line from refreezing until you get to fixing it properly.