RX pin problem

Hello all,
I have a problem with my UNO board concerning the serial pins.
I have searched the web for this particular problem but haven’t found anything related enough.

Narrowing down and pinpointing the problem, I have found that when my RX pin (D0) is connected to another device’s TX, it makes the board’s procedure stuck.
I’ll explain:
Uploading software works fine, so its not the receiving part that’s gone bad.
If I send chars through the IDE serial monitor, the program reacts fine (printout) if the RX pin is not connected to anything / connected to random pins (including to other device’s RX )/ connected to 5V with appropriate resistance. Which, ofcourse, is practically useless for my needs.

BUT, if my RX pin is connected to any device’s (directly from Arduino Mega or through an Xbee adapter) active TX pin only, the program won’t react whether or not the USB is plugged in.
No other connection has any other effect on the program besides that.

I do not recall having this problem last week. Could it be that I somehow ruined the pin?
or did I do something else which I did not notice?