No sex in 3 months- advice please

Me and my SO have been going out for 3 years now, and at the start sex was all the time, we still live with our parents but we always found a way to 'do it'. When they left the house for 5 mins we would have a quickie, when they left for the day we would go back to bed or head for a long shower. But now sex can be months apart, 3/4 months apart, we are still madly in love with each other and have been through soo much together, but I can't understand this! Even when we were away for the week with our own private apartment- no sex, not even foreplay. Any suggestions how I can encourage it more often or spice things up a bit? Thanks