I suspect you have had a thick covering of snow for long periods during winter. This is the most likely cause. The best course of action is firstly, give the whole lawn a very light raking - not to hard at this time of the year. Next, spike it all over, followed by a light mowing with blades set high. towards the end of this month you must feed the lawn with food containing a high rate of nitrogen. this will rapidly green your grass. Hope this helps.