Temperature
During a cold wet winter, your lawn tends to go dormant and stops putting on any growth. This does not mean that it has died, only that it is resting and in order for your grass to spring back to life it needs warmth.
Whether you are sowing new lawn seed, overseeding or waiting for the existing lawn to get growing, without warmer soil temperatures not a lot will happen. This does not mean that the grass won’t grow or that the seed won’t germinate, it just means you have to be a little bit more patient, and that as soon as the weather warms up the new grass will start to grow.