Why won't the HD channels come in on my lg flat screen monitor? We changed the cable box and everything. Any suggestions?

The first thing you need is the HD tuner. Since you stated that you changed the cable box I will assume that means it's HD ready.
So you have HD signal coming in to the cable box box by-way of a coaxial cable. Now you need to get that HD signal to the monitor. For this connection the standard coaxial cable normally used between the TV and cable box will not do the job. How many connection options you have depends entirely on the what is available on the monitor.
Normally there are three choices. Your monitor could have anyone of these, a combination of them or none if it's not HD ready.
They are Component cable, HDMI and DVI.
Component is a combination of three RCA type (red, blue, and green), connectors. Simply connect the "Component Out" from the cable box to the "Component In" on the monitor.
HDMI cable looks similar to a computer monitor cable but bigger. Same goes for this one. Connect the "HDMI Out" from the cable box to the "HDMI In" on the monitor.
An indirect connection can also be made if your monitor has a "DVI In" jack by using a cable that will allow you to come from the "Out" connection of either the HDMI or Component on the cable box to the "DVI In" on the monitor.
If you are unclear about anything or you have any other questions please just ask and I'll try and help out.