Serial monitor problem when connecting external circuit...help?!

Hiii
I am having problems in the serial monitor when connecting my circuit to the Arduino Pro-micro. A couple of days before this, I could connect my circuit and could not see the data in the serial monitor but then I connected the GND pin from the Arduino to my circuit and it worked fine, I try it for about an hour and then the day after and it worked, but when I try it today I could not longer see the data in the serial monitor even if I connected the GND pin like I did before. Can someone tell me or give me a hint of what could be the problem now?

Thanks!

Can you share your current Circuit diagram here.
I am trying to understand Whether you directly monitoring using Serial monitor or using USB to TTL device to monitor the result

I am sorry, I don't have a full diagram of my circuit to post it here but what I received from my circuit is a pulse that activate my external interrupt ( 4 of them) from the Arduino Pro-micro.

Explain problem statement properly. What your are trying todo. Is there any 2 Serial communication in your project.??

what I received from my circuit is a pulse that activate my external interrupt ( 4 of them) from the Arduino Pro-micro.

what your are receiving???
from Where your are receiving??
what are Device arduino board being connected???How Arduino board powered here mean using usb your using power supply.
If it power supply make ensure all are at same ground potential.

I am receiving cuadratic pulses from my external circuit ( a peak detector). I try powering the arduino externally with the same power supply connected to my peak detector and then connecting it to the serial monitor via USB and also try powering the arduino with the USB and connecting the GND pin to the peak detector. In both cases and still can't see my data (duration of pulses) in the serial monitor.

Try basic thing
You just power on arduino and peak detector circuit using USB By disconnecting the external power supply,Then open your Serial Monitor . This time it should work.

Later i will tell What you need to make it work with power supply.

What voltage level are the pulses and do you need/are you using current limiting resistors. Really need more information and a schematic else this could end up 20 questions.

NOA2:
I am sorry, I don't have a full diagram of my circuit to post

A photo of a clear pencil drawing will be fine. It will be hard to help without it.

...R

AMPS-N I did already but I did try it again just in case I was making a mistake before but it didn't work.

Sorry for that. Until and unless you put circuit diagram we cant do anything.

http://www.expresspcb.com/expresspcbhtm/download.htm
http://www.cadsoftusa.com/