Make sure you have easy access to your mains valve just in case, then hold valve body in a pair of grips without crushing it and then a large flat driver that you can give a good twist on and hope, they normally move but will probably drip thereafter so a repalacement is needed.
Best advice I can give is to not buy the cheap ones that are often on a multibuy special like these
Isolating Valve 15mm 2 Pack
These are more expensive but fit and forget,
Pegler Isolating Valve 15mm only thing to bear in mind is they are not full bore so if you want full bore then
Pegler Tee Ball Valve Blue 15mm and for something with a lever
Pegler Ball Valve Blue 15mm
The cheap ones jam and leak which keeps your plumber happy in work but not your bank balance. As for your main water valve if it is a gate valve or older style T valve then change it for a lever valve because they only need to move 90 degrees to shut off which if you have a big leak you will find very usefull, seen many people in this situation where the main valve has not worked and they have an unwanted indoor shower.