Basically you need to keep your BS levels slightly raised, so that your body gets used to not being hypo for a while. Then your brain sees it as a real emergency if they drop below normal, so that you get your warning symptoms back. At one point I suffered, and I do mean suffered, so many hypos that I had this sort of problem. The DSN didn't want me to cut my insulin dose and that was the cause. I then dropped my basal insulin 2 units and that largely fixed things. I decided it was my life not hers. I still had to eat extra sometimes to avoid BS levels falling to below 5 for the next couple of weeks. I had been told it would take about a monrh to 're-set' my brain, but it happened for me in between 2 and 3 weeks.(5 was my decision as the lowest level I was happy with.) I also avoided any extra exercise, which I knew would make it plummet.)
I also lost hypo symptoms when I was pregnant, but 5 days on a glucose drip with BS levels closely monitored sorted that out. I was discharged after an emergency Caesarean op. only 1 day later than others without the type 1 problem, with hypo symptoms regained. I had to be quite careful for a while, eating exactly on time and doing extra blood tests but the drip had done the majority of the work, if you see what I mean. You can get warning symptoms back, but it is sometimes a bit of a battle. (This was 25 years ago.)
I think it also helps if you don't feel unwell for any other reason. If your warning symptoms start with general fatigue/ malaise, it may make you reach for the paracetamol rather than glucose. If you feel not right in any way, you need to go for the testing kit first, but have the glucose in grabbing distance.