Have a bowling machine session with balls set on a good to full length on your pads. Even though it’s a machine the line will still vary slightly. Try and hit everything back past the machine in the V, ideally to mid on. Start easy just getting a bat on it and progress to full blooded shots. At some point in the session something will click and you will murder it through midwicket/square leg (depending on your personal way of going about such a shot)
It can take ten minutes or 2 good length sessions, but persist with it.
As you’ve always been fine I suspect it’s not a major technical problem. I’m saying that without the advantage of seeing you before and after this problem of course but I’d hope your mates would point out a major stance or technical change. The advice about having a look at your head alignment is good but to be honest everyone’s head moves to the offside to some degree or another, it’s inevitable. If you are really falling over badly then as mentioned your teammates should have pointed it out.
I think of the shot as quite an instinctive one rather than a set list of technical steps to follow. Some people have that impressive ‘pickup’ over square leg that looks effortless, others seem to be able to almost middle it to fine leg. When speaking to players who are strong on the legside their feedback seems to consistently be along the lines of ‘I don’t really know what I do, I just hit it’
I’ve done the above exercise dozens of times with customers and it works, it’s like a lightbulb moment, after farting around for a while, then hitting them nicely through mid on, they will nail some into their preferred legside area and then I see the look on their face. ‘Oh. I CAN hit it there’