Breathe out, relax, check the horse if they've sped up, and keep riding as you try to scoop your stirrup back up. If you feel like you're going to fall though, stop. If you're like me and you lose your stirrups a lot (I've got weird feet and it's super hard to find stirrups that fit properly), do a lot of no-stirrup work under a trainer's supervision to build up the muscles you need to ride through it.
I think that if you loose your stirrups, it means that you don't relax the leg pressure. By that i mean that your knee is probable too tight to the saddle and you don't put enough weight on your heels, you are probably also too tense on your horse's back.
To let down your leg, you can work without stirrups or try to be standing on your foot.
You can also buy stirrups with a large floor or articulated for more stability.
Hi Nisha, I just keep on riding, I relax my lower leg and point my toes up to try to catch my stirrup again. When jumping a show it's a little more difficult, there I try to slow down a little in a long line or turn, but if that doesn't work I keep on riding without stirrups. Had enough practise in that with my trainer 😂 If losing your stirrups happen often, ask your trainer for exercises to relax your upper leg, and some no stirrup work. It's a pain in the ass in the beginning but it's good for you!