4 ATP (net gain: 2 ATP)
In theory a cell can still produce 4 ATP when the electron chain is inhibited. In the processes before the electron transport takes place, ATP can still be produced.
The image below shows that during glycolysis 2 ATP is produced and 2 more are produced in the Krebs cycle (citric acid cycle). In the first steps of glycolysis there is an investment of 2 ATP, so the net gain would be 2 ATP.
The other ATP will not be produced, because for this the electron transport chain is required: NADH hands over the gained electrons to the proteins in the electron transport chain.
However, the inhibition of the electron transport chain is so toxic for the cell that also this production of ATP will stop. There is a build up of intermediate products and reactive oxygen species are produced. If the inhibition continues, the cell will inevitably die.