To make whipped cream you need to use cream that has a relatively high fat content, because it is the fat that enables the cream to expand and thicken. Whipping the cream introduces air in the form of tiny bubbles, and the fat molecules cling to the outside of the bubbles. The proteins and lactose in the cream get combined between the air and fat molecules, making the cream stiffen. After a certain period of time, the air bubbles start to burst and the whipped cream breaks down, returning the cream to its original runny consistency. If you find that this happens, it is easy to re-thicken the cream.
Put the cream you want to re-thicken into a mixing bowl you have previously chilled in the fridge. It's important you keep the cream as cool as possible.
Gently stir the cream using a cold spoon. This will combine the constituents in the cream, as some separation may have occurred since you last whipped it.
Add a couple of tablespoons of fresh cream to the bowl and stir gently. Although this is not essential, it does help in the process of re-thickening cream.
Set the electric whisk to a slow speed. Turn it on and start to whisk the cream. It will initially get very runny. Increase the speed to medium and whisk until the cream starts to re-thicken. Gradually pour in 2 tbsp. of caster sugar while continuing to mix. You can use regular granulated sugar if you don't have caster sugar.
Turn the whisk to high and let it whisk the cream until the cream re-thickens. It will form soft firm peaks when it's done. Turn off the whisk and return the mixing bowl to the fridge until you want to use it.