Starting an old Fluval can be tricky, depending on how it was stored. You should disassemble the unit and examine the impeller, and impeller well. Often a filer is put away right after it is drained, without cleaning it. If that is the case, there may be some caking in the impeller well that keeps the impeller from spinning. remove the impeller and clean any dirt or debris in the impeller well or on the impeller itself.
Once you have done the cleaning, run your finger into the impeller well and ensure the sides are smooth and even. If the impeller well has lost its smooth walls, it may have been run without water and the plastic has deformed. See if the impeller will spin by hand, there is the chance it has been so deformed it will no longer spin. If this is the case, then it is probable the unit will not run again.
If the impeller well does appear to be clear of deformity and the walls smooth and the impeller turns by hand without too much resistance, reassemble the impeller into the pump motor and clip on the impeller cover - then plug it into power and see if the impeller spins by the motor. If it does, it should be able to pump water.
If you have an older model, and you know the impeller is spinning freely when plugged in. I have found that the best way to start the unit is from an empty state. Get the water flowing into the filter from the input, either by sucking on the hose (for the old models) or using the self primer to start the flow, just make sure that the unit is filled from empty. As the water flows in, plug in the power to make sure the impeller is spinning at full speed when the water reaches it so there is no chance for the system to air-lock.
If the filter starts, but then stops later on, chances are the filter media, especially the carbon, was not rinsed and submerged long enough to eliminate the air trapped n the media. It will release as the filter runs and gather at the top of the filter. It never hurts to shake the filter a few times over the next few hours after starting to help release the air and keep it running.