Dave makes a very good point (that we seem to make here all the time). The more information you can give us, the better chance we have to help you.
It *does* sound like something is blocking or interfering with the signal, but that might not be the true problem.
When did the problem start?
Did *anything* change in your home around the time the problem started? Think about *anything* that you might have plugged in, that wasn't plugged in before (new cellphone charger, laptop, TV, surge protector, etc).
Did you move anything from one outlet to another, anywhere in your home?
Did you recently install or replace any light bulbs (CFL or LED) anywhere in your home (not just in the lights controlled by X10)?
What type(s) of modules do you have?
What type(s) of bulbs are you using with them?
What model interface are you using (CM19, CM15, or CM11)?
You mentioned the lights won't turn ON. If you turn them on manually (at the device itself, NOT in the software), can you turn them OFF from within AHP?
Let's start there, and see where it goes.