Hi @dragoid22
It could be just the fuse, but if it has failed then usually there's a reason why as fuses normally don't fail of their own accord.
Below is an image showing the location of the main AC input fuse on the power board of the display.
It is a T3.15A 250V sub miniature slow blow fuse (supplier example only).
You could try replacing the fuse but don't be surprised if it blows again as soon as the power is connected or shortly afterwards. (buy 2-3 fuses, just in case)
If this happens you would need to check the power board for any obvious component damage or dry joints on the soldering side (heat marks?) that may be causing this to happen and if nothing is obvious the schematic for the board would need to be found to work out what the problem is. I couldn't find it, you may have better luck.
In the end it may be easier, given the price of the power board in the link above, to simply replace it. Search for Dell S2421NX power board to find suppliers that suit you best.
Here is the Dell S2421NX simplified service manual that shows how to disassemble it so that you can remove/check/repair or replace the power board.
(click on image)
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