I had flickering lights in bathroom and hallway for some time. Then it stopped but they
Started to flicker again and now don't work at all. Removed switchplate and wiring looked OK. He went to basement to check circuit breaker and found it was very loose. Is this a problem for an electrician? Does this happen to circuit breakers?

Yes a breaker can be loose as it plugs onto a tab in the panel. While the breaker may just need to be replaced, the tab it plugs onto needs to be checked to be sure it is in good condition. It should be , but if it is burnt or discolored from overheating, the breaker should be moved to a spare slot, and the panel may need to be replaced also.

If your not knowledgeable working in a panel, you should have an electrician repair this.