Initially I could run a super bright 3MM led with 3V on Arduino Nano, after I tested it several times suddenly the led dimmed but when I tried it on 3V external the led was normal. Is there something broken on the 3V line?
If you did not have a current limiting resistor in series with the LED, then you damaged the Nano.
You have probably drawn too much current from the Arduino board and damaged one or more of its component
Arduino boards are not designed to be used as power supplies
Please post a link to the product because there is no official 3.3V (3V) Nano. If it's a 5V Nano, please provide a wiring diagram.
What does the 33 indicate in the name of the Arduino Nano 33 IoT ?
Asking me? OP did not mention which Nano so it's by default a classic Nano 
Or did I miss it ? 
Yes, in response to your assertion
Blame "Arduino" for muddying the waters
I have done as you did only the 3V on the arduino nano 5V is not working normally suddenly
Hi, @bimosora
What do you mean?
What code are you running?
To add code please click this link;
Thanks.. Tom.. 
What value resistor did you use?
If you have a typical Nano clone, the 3.3V pin is actually the output of pin 4 of the CH340 UART adapter on the bottom side of the Nano. It is on whenever the Nano is running, whether connected to USB or not. It doesn't provide very much current, and if you powered the LED without a resistor, there may have been enough current draw to blow out the 3.3V drive in the CH340. If the CH340 is otherwise ok (you can still flash sketches to the Nano), you can continue to use the Nano normally, but without any 3.3V output. You could power the LED from the 5V pin, but with a resistor of course (470R to 2.2K maybe).
Anyway, if the Nano is powered up, you should be able to measure 3.3V on the 3.3V pin with your multimeter if the CH340 has not been damaged and if the traces from it to the pin have not been damaged.
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