Like Marketing, Sales, Manufacturing, customer service, Finance, HR too is an evolving subject. Every day new concepts, new rules, new regulations coming up. Much depends what was your break period like. There are sites like sightinplus where articles keep appearing. Downnload and read. What was your core areas of involvement prior to your break ? There are a lot of subkills in HR - In between Recruitment and Retirement or Hiring and Firing - Competency Mapping, HR Scorecard, Performance Management System, 360 degree feedback, Employee Engagement, Counselling, Industrial / Employee Relations..... HR Business Partner role, HR Data Analytics....... The list is pretty long.
Best wishes