when you say gone into lost/stolen mode. Did you phone Virgin Mobile, report the phone lost & they've blocked it?
If this is the case, give Virgin mobile customer services a call on 789 from a Virgin mobile or 0345 6000 789 from any other phone, tell them you've now found your phone (you'll need to know your account password & possibly the phones IMEI number) & they should be able to unblock it for you.
If you want to say thanks > click 'Kudos'. Answered your question? > click 'Mark as Helpful Answer' also.
if you didn't report it lost/stolen to Virgin, then they won't have blocked it.
Does it still have your Virgin SIM card in it, is that still active?
What exactly does it ask you for when you turn the phone on?
Did you set your own security pin on the phone & then forget it? The default is often "0000" or "1234" but you may have changed it to... your birthday.... favourite year.... house number... - Is this the case?