How do I troubleshoot my pool filtration system?
Let us start with some basics of how the pool plumbing works. This picture helps to show the basic connectivity of a typical swimming pool system.



(Picture borrowed from website)

Note that not all pool setups have two separate pipes to draw water from the skimmer or main drain and bring it up to the to the pump station as shown above. On some the main drain T's into the bottom of the skimmer and then a single line goes to the pump. The following picture shows this alternate type connection and when one wants to drain the pool the skimmer intake is blocked via a diverter in the bottom of the skimmer. 



(Picture borrowed from here)

The above picture shows the basic plumbing circuit for water flow when the pool is operating in the normal filtration mode. Not shown are the various valves that are used to re-direct the pump water flow backward through the pool filter to suppport the back flush operation. Here is yet another picture showing a setup with all the valves. 



(Picture from here)

In the above the numbered elements are as follows:


Swimming Pool Skimmer Line
Swimming Pool Main Drain Line
Swimming Pool Slide Line
Automatic Pool Cleaner Line
Swimming Pool Return Line
Swimming Pool Return Line
Automatic Pool Cleaner Motor
Auto Sanitizer
Swimming Pool Heater
Swimming Pool Pump
D.E. Pool Filter
Pool Heater Gas Supply Line


Hopefully that helps to gain an understanding of how these systems work. Now let's talk about what could be wrong with the setup that you have. 

a) Make sure that after the back flush operation that you have returned all the valves to their position for normal filtration operation.

b) Most pool pump assemblies are equipped with a debris basket on the inlet side of the pump. Make sure to clean this out. If it gets all full of leaves and other stuff that comes from the pool then the pump will not be able to draw water from the pool. 

c) Make sure to clean out the basket assembly in the pool skimmer. Generally when this gets completely full of leaves, tree droppings and flowers there is greatly reduced water flow in the system and the pump will be unable to produce much pressure. 

d) Also check to make sure that the skimmer pipe in the bottom of the skimmer is not clogged up with debris that may have pulled through a broken skimmer basket. If the basket is broken it should be replaced. 

e) Now you can turn on the pool pump motor. When it is running you should be able to observe suction in the bottom of the skimmer. If there is no suction there then there is either an improperly positioned valve, the line from the skimmer to the pump is plugged, the line is broken, the pump intake basket is plugged or the pump itself has failed. 

f) After you have the pump going again it is necessary to purge air that gets trapped in the top of the filter assembly. You should find a air bleeder valve near the top of the filter (may be part of the pressure gauge) that you open to expel the air. When all air is removed water should shoot out of the bleeder valve. 

g) Be aware that when the DE filter has been flushed most of the DE material in the filter gets ejected out into the drain line. No filtration can happen when returning to the normal pumping mode until fresh clean DE is introduced into the system via the pool skimmer. Follow the directions for your filter to know how much DE to place back into the system.

Hope that helps with the basics. There are obviously other things that could go wrong but start with this and comment back if more information is gathered.