Why has one circuit lost power?
Simply put, you have a true short in the cabling someplace. To solve your problem:  First of all be sure you are working with the affected outlets and fixtures and don't accidentally pull something out that is still powered up.  Then you should pull out all the affected outlets and fixtures and look for any shorts. If none found, take pictures of each one BEFORE. Then disconnect everything including the connection to the breaker.  Be sure that you aren't testing anything live by testing for voltage first.  Then start testing with a DMM on the ohm setting.  You should have near infinite resistance across the white and black in every case. If you find a short (very little resistance), you should be able to find the &quot;other end&quot; of the cable because there should be another pair with low resistance.   Then, hooray, you found the cable with the short. What you do from there depends completely on where that cable runs, hopefully not to difficult a replacement.   It would be good though, to identify the cause of the short. It's very unusual for the cable itself to be compromised.
But rodent damage can do it:  I had a similar situation in my previous house.  Some outlets and lights stopped working and another circuit started tripping breakers with nothing plugged into it. The affected outlets were on a wall shared with an attached garage. So I started pulling off sheetrock and this is what I found!