Earlier I used to do regular pushups about 30-40 and was able to perform about 5 chinups everyday. I left training for a while and then for the past few weeks started training with 10kg dumbbells. About 40 hammer curls and another 40 unilateral curls once or twice a week. Now I definetly have more defined upper arms than before but I can't perform even a single chinup even 4-5 days after workout. I don't take supplements but do believe I have my fill of protein from eggs etc. I don't feel any damage in my muscles or any other hamper so can't understand the reason. My weight have been constant throughout ie 65kgs.
I'd like to break down a few things first which I think might help to explain what I think is going on with your situation.
Bicep curls on the other hand are maybe not the most absurd exercise, but when you combine their popularity, the various types of them, and the countless gym-goers that spend inordinate amounts of time curling, bicep curls really do take the prize for silliness.
Exercise and adaptation is specific: you spent a few weeks (the time since your last chinup) not exercising your back in any great degree (based on what you wrote), so it makes sense that your body would have realized it doesn't need that strength anymore.
The "normal" amount of chinups/pullups that are recommended for a young-ish healthy male is about 3 sets of 12 reps. Do 12, wait a minute or two, 12 more, rinse/repeat until you get to 3 sets.
I would highly recommend ditching your curls and spending more time back on the simple pullup bar. I haven't (happily) done a curl in over five years and my arms are plenty big enough. The secret is weighted pullups. Once you can do your 3 sets of 12, try to do 3 sets of 5 weighted. As much weight as you can handle for five reps. Every week, crank up the weight a tiny bit. Favor chinups (with your hands facing you), as they tend to target your biceps a bit more.
The pullup/chinup is, quite frankly, a superior all around exercise that does more for your body, has less chance of causing elbow tendinitis, and no one will confuse you with being one of the know-nothings just curling away watching ESPN in the gym.
If you work your way up to doing chinups with 90lb's of plates dangling from a chain, your arms will look like bazookas and you'll have never needed to spend a minute doing curls.
If you could do 5 chin-ups 4 months ago, then didn't work out for three months, then you started doing curls, I think you just got weaker from not working out.
Curls won't necessarily improve your pull-up performance dramatically. 10kg dumbbells are way, way lighter than you are, for example.
If you want to be able to do pull-ups, do pull-ups. The definition in your arms has very little to do with your ability to do them.
Would definitely agree with Eric, and I would have added on to his comment but I'm new on here :)
If you're isolating and smashing your biceps several times a week you should ask yourself if you're giving yourself ample time to fully recover - I had a similar problem in years gone by but I was also making it worse because my body was building up a kind of tolerance to exercises that I was repeating over and over again. Essentially I was becoming better at performing that exercise, but it wasn't giving me an overall benefit. That's why I advocate compound exercises, or mixing up routines - I won't just focus on my biceps for 30 minutes.
A couple more things...
While chinups do use chest you're mainly targeting your arms (triceps and forearms as well as biceps), your shoulders and your back. It could be you've gained mass in your chest and arms and you need to compensate for that - you're heavier than you used to be. I find chinups as well as dip reps can fluctuate a lot depending on how heavy you are, whether that's muscle mass or fat.
Try doing some seated and / or upright rows to strengthen your shoulders and back or, if you want to burn fat and increase your cardio resistance, jump on the rowing machine and alternate the intensity.
Also don't obsess over protein too much - obviously it's important but good quality carbs are so often neglected these days. You're going nowhere without decent carbs - that's the fuel powering your workouts.
Supplements do help, but nothing will beat a good quality diet.