If there really was only one tank of fuel available on an aircraft that typically has two tanks filled, that would at a minimum mean that the airplane would have to fly on half the fuel that was intended to be available for the rest of the flight.
If such a failure happened mid-flight, as in your claim in the question, it'd mean the pilots suddenly had only 25% (of what they took off with) usable fuel, when they intended to have 50% at that point in the flight, having taken off with 100% and expecting to land at near empty. In such a case, unless you were very near the destination anyway (in which case delay would be insignificant, so no real need to tell anyone in the cabin), to not divert to the nearest suitable landing site would be irresponsible on the part of the pilots.
Since you say that the flight completed without any noticable events, it stands to reason that the flight indeed did not divert.
The only reasonable exception to the decision to not divert after half the remaining fuel becomes inaccessible mid-flight would be if the pilots took off with at least 1.5 times the fuel they needed for the flight.
Let's say that the flight required 100,000 kg of fuel, plus applicable reserves. Normally, half-way through the flight the aircraft would be down to half that, or 50,000 kg plus reserves, distributed evenly between the tanks. Assuming two tanks, that's 25,000 kg plus half a reserve, per tank. If only one tank is available, but 50,000 kg plus reserves is available, there must then be no less than 100,000 kg on board (50,000 kg of which now unusable) after burning through the fuel needed for the first half of the trip (corresponding to 50,000 kg of fuel). Therefore, the aircraft would have had to take off with 150,000 kg of fuel plus reserves, 1.5x of the required 100,000 kg plus reserves, to still be able to make it to the intended destination with the same amount of fuel remaining as intended if half the remaining fuel becomes unusable at the mid-point of the flight. Since hauling fuel around itself requires fuel (because the aircraft is heavier), pilots don't load up amounts of fuel above and beyond what they need, without very good reason. Assuming that the aircraft was topped off with an appropriate amount of fuel before take-off, the pilots would suddenly be faced with having at most half the remaining range intended; likely less, because you don't run the tank all the way down to empty, especially when the other wing is weighed down by at least 50,000 kg of fuel.
Therefore, I conclude that there must have been something going on beyond what you describe in your question.
There may have been an issue, and it may have had something to do with the amount of fuel available to the engines, but it seems highly unlikely to have been the sudden inability to use half of the fuel remaining onboard.