What to do? Best friend that lost their job and won't talk to me while I'm starting new job.

It sounds like they are jealous. A lot of times the way friends act when they’re jealous can make you feel horrible. It's good that you know the reason that they are not talking to you, this way it may be easier to resolve. Reflect on how you could be contributing to their jealousy. While your friend may be happy things are going well for you, if they're having a rough time they may find it harder to be openly encouraging. Try to keep in contact, even if they are not willing to keep in touch, show that you care by being persistent. Maybe send them text messages to cheer them up. What would you want them to do if you were in their position?