12 volt system has stopped working

I plugged in 2 phones plus the tv and sat nav finder into the 4 12 volt system the previous owner put in for tv and satellite dome.
I think I have blown something as I have no 12 volt appliances working.
If anyone could advise if it’s a fuse or whatever and where it’s located I would be truly grateful.
The battery is under the drivers seat.

You state the “12v system the previous owner put in”.
If I’ve read it correctly, that means that it won’t be anything that anyone will be able to advise on reliably as the previous owner has dabbled with the electrics.
Nothing too difficult I’m sure but without seeing the install who knows?.

Try a pic....
Then we might know where to start??

The previous owner set up a 3 socket cigarette lighter through the 12 volt system this has worked well for over a year.I plugged in 2 phones in one usb plug  and the tv and sat dome were plugged in to the 2 vacant sockets which has been working without any problems until I plugged the twin usb plug into the vacant socket.we had tv and sat done box and 2 phones working all at the same time for about 30mins then everything just stopped.no 12v at all.
Sorry if this reply is not in the correct place as I’m not good with this site(silly old fart) and computers.
Thank you in advance.

Try this.
Remove the multi adaptor.
Plug in just one item previously powered by it directly into the vehicle socket.
If that works, the adaptor is at fault.
Do that with all you wish to power.
If all works then the adaptor is faulty, if none works then the socket isn't powered.
NOW some are only powered with ignition switched on, try that.
If still nothing, there is a fuse powering it. Find that and change it
Back to the adaptor, in some the pin which makes contact in the socket unscrews. If it does there will be a fuse in there, change it if you can get one, or, simpler, bin the adaptor and buy a new one, they're very cheap!

All the 12v system has stopped working so I think it must be a fuse but where would this be situated.I have no feed from the 12v .

You are going to have to check all the fuses you can find, also use a multimeter if you have one with you to check the fuses, you have got a fully charged battery haven’t you as it’s the simple things first