What happened to your old school/university/work colleagues, old and present neighbours, mums from your child's school, people you met on holiday etc. Did you lose them along the way? However, it's never too late to start again. Friends have to be cultivated, favours done for them, outings, experiences and discussions shared.
Walking the dog is a good place to start,volunteering in charity shops, theatre groups always need people and not just for acting, hearing children read in school, helping at mother and toddler groups in church halls or Brownies, joining clubs or local political parties, ramblers, reading groups, library activities, singles holiday companies and so on.
The list is endless and charities and church groups will never turn your help away (you don't have to be a believer) but no one is going to come knocking on your door begging for your friendship. Be well informed about current affairs/what's popular on tv etc. so you have plenty to talk about. Invite your neighbours round for a cuppa.
Do you smoke? I always cross smokers off my list, I'm afraid, when meeting new people no matter how interesting they are as I can't stand the smell of stale tobacco on their clothes near me. Are you a Moaning Minnie? Think about why you have no friends and set out TOMORROW to change your life and find some!