Half of electrical circuits remain out after exterior neutral failure
If 1/2 work probably L1 or L2 leg on the supply side or main bkr has failed this will cause every other bkr on the left and every other on the right to not have power. I replaced a 200a main a few years back that had this problem although I like square d this was the first time I have seen this and 1 more from Eaton since then. The power from the feeder needs to be checked prior to the main and after. Sounds like you have an Electrician doing some of the work but some states in the U.S. Have very lax rules and not much training. If one of the legs have failed that may account for 1/2 of things not working. Do you have 220? Like for a dryer or electric range? If not your power to the new panel is probably the issue.