Simply transfer the SIM card, that has the credit balance, to the new phone. The new phone will then have the existing number and credit balance. You might have to get the new phone unlocked if it is currently locked to some other network
If the SIM card in the broken phone is not working or is the wrong size then ring Virgin Mobile and request a replacement. Usually sent free of charge.
To Mat-H - Yes did get in touch-old sim tooo big for new phone (is Micro sim) so purchased new micro sim 99p and set with new phone no. also got old phone money left transferred to new phone - A good result all round .