How do you trace a dead hot wire backwards if you have several lights and plugs not working on a circuit and you have opened every box nearby looking for a loose connection without luck?
== == (LIFE SAFETY WARNING! [disclaimer]
Electricity is dangerous!
You can be injured or killed!
Improper installations can cause fire, injury and death!
Should you be doing this yourself?)
The only reason there is "not enough voltage" somewhere is because there is a partially or completely open connection providing resistance and a location to allow a voltage drop [bad splice, bad switch, bad breaker, broken wire, burned splice, ...].
It is the knowledge of how electricity works, and of the methods and materials used to create a functional wiring system, that enables a skilled troubleshooter to locate the problem and repair it.
Where should the "voltage" be, and how does it get there?
When you understand that, you will understand what is keeping it from getting where it should be...
What are the possible routes from the items that don't work back to the panel?
I would suggest that there is something on this circuit which still has power, but which is not allowing it to continue to the rest of the circuit.
There may be a receptacle at the other end of the house which works but has a burned connection to the outbound conductor.
LIFE SAFETY WARNING! [disclaimer]
This information is provided for the use of parties as they see fit!
TURN THE POWER OFF!
You are more likely to be killed by 110 volts than any other. It can create PERFECT fatal current through human body's electrical resistance! <><><>
As always, if you are in doubt about what to do, the best advice anyone should give you is to call a licensed electrician to advise what work is needed.
If you do this work yourself, always turn off the power
at the breaker box/fuse panel BEFORE you attempt to do any work AND
always use an electricians test meter having metal-tipped probes
(not a simple proximity voltage indicator)
to insure the circuit is, in fact, de-energized.
IF YOU ARE NOT ALREADY SURE YOU CAN DO THIS JOB
SAFELY AND COMPETENTLY
REFER THIS WORK TO QUALIFIED PROFESSIONALS.