Just found out someone important in my life has lied to me, I'm hurt & a bit upset. What should I do?

I've been through this. You are certainly within your rights to challenge them and to let them know how you feel. If you feel like I did, this has caused you to rethink and reconsider other things they've told you in the past and caused you to question whether you've been lied to about other things. This person needs to understand that, and they need to understand that these lies have damaged how you feel about them. It's a tough spot to be in, but I think the best you can do is to combat the lies with honesty.