I'm pretty new to Arduino but I've got a grasp to most of its feature.
I'm making a campus project on the topic of simple digital thermometer. I used an independent microcontroller (moved it to the breadboard) which is ATMega8. The thermometer works, in the point of the LED managed to turn on since everything is connected correctly. But whenever the RX or TX pin is not used(e.g no jumpers are connected) hitting reset would stop it from functioning (LED would not turn on again).
You're providing not that much information, so I'm guessing. Leaving the RX/TX pins floating is not a good idea with an Arduino bootloader because after resetting it looks for traffic on the serial interface (it interprets that as uploading a new sketch). A floating pin is jumping from HIGH to LOW and back in a random fashion and that's easily interpreted as serial traffic although the traffic don't make sense. So if not used it's best to jumper TX/RX to GND over a resistor. I don't know if you've done that because you didn't provide a circuit drawing or schema of your board setup.
After consulting with my lecturer about this "floating" state, we managed to fix the circuit by either connecting jumper cables to either a ground or a 5V source. Thanks for the advice!