Unfortunately ESX 3.5 used a license server which is usually installed on the vCenter Server. To avoid the issue you are most likely facing, there's a 14 day grace period, which however does not always work as expected. What you may need to do in this case, is to generate a "hosted license" in the My VMware portal (I hope this is still possible) for the ESX host on which vCenter Server is currently registered. After assigning the license to the host you should be able to power on vCenter again.
André