So you will always get periods of time when your baby/toddler/child doesn't sleep very well. This might be because of teething, illness, development or when older because of things like nightmares, poor sleep hygiene... I could go on and on. There will be loads of reasons your son will go from sleeping well/easily to sleeping rubbish. You'll never avoid this, all children go through these regressions several times over.
The problem with sleep training through controlled crying is that it's an all-or-nothing approach. So baby went from (presumably) having lots of your comfort in order to get to sleep, to needing literally none of your comfort.
That's fine for actually teaching children to sleep independently, as you've found, it works. The problem is when you face one of the million reasons sleep goes rubbish.
If you'd done a gradual reduction of the comfort you give, it's a bit like a ladder. You make steps towards independent sleep and will eventually get to the point when you put baby down and leave. But achieving that was dozens of small steps. So when you get a regression, you just backpeddle a few steps to give a bit of extra TLC but you don't have to go all the way back to square one.
With CC, you have no middle ground established for giving the little bit of extra comfort DC needs - hence it's all-or-nothing. You end up going back to where you were at the start.
So to solve it, two possibilities:
(1) with every regression, like you're finding yourself in now, you give the extra TLC needed and then have to redo Controlled Crying. It should be easier to establish than it was originally, but it does mean going through this all over again. Plus, expect to need to do it fairly regularly - there's always reasons DC need extra TCL in the short term.
(2) instead of a CC approach, use a gradual withdrawal approach. This involves small steps towards your end result. It takes longer than CC to see the same result, but arguably over the long term is less hassle because at small regressions you can just go back a few steps to give extra TLC, not back to cosleeping or whatever.
That's all to do with the way you deal with the regression you find yourself in, what you choose depends on your parenting style. What I've not mentioned is the reasons for the regression.
At 10 months old, the most likely reasons are teething pain, motor development as baby learns to pull to standing (This almost always makes sleep go shit!), or over tired because daytime sleep routine need to change.