user6294: I had my sketch uplaoded and everything was working fine when I tried connecting one of the input pins(pre-configured as digital INPUT) to 12V, just to see if this was being read as a HIGH as well.
It seems to have done something and the board stopped working. Moreover, all pins seem to be stuck at HIGH! 
So far Ive tried the following:

reset button: doesnt help
re-upload: doesn't work - it says not in sync

Physically nothing seems to be burnt/blown. What am I missing? Have I fried my board? I hope not. PS: It was connected to a 12V external power source
Any thoughts?

user1585: Unfortunately, the arduino's input pins are not tolerant of 12 volts, You have likely caused permanent damage to the chip =(. There is a regulator that makes it capable of being powered by 12 volts, but the input pins are different.
If you look at table 30-1 on the datasheet for the Uno's controller here you can see the input characteristics. The arduino is powered at 5v, so the max digital input voltage is 5.5v

6294: <p>I had my sketch uplaoded and everything was working fine when I tried connecting one of the input pins(pre-configured as digital INPUT) to 12V, just to see if this was being read as a HIGH as well.
It seems to have done something and the board stopped working. Moreover, all pins seem to be stuck at HIGH! </p>

<p>So far Ive tried the following:</p>

<ul>
<li>reset button: doesnt help</li>
<li>re-upload: doesn't work - it says not in sync</li>
</ul>

<p>Physically nothing seems to be burnt/blown. What am I missing? Have I fried my board? I hope not. PS: It was connected to a 12V external power source</p>

<p>Any thoughts?</p>
 1585: <p>Unfortunately, the arduino's input pins are not tolerant of 12 volts, You have likely caused permanent damage to the chip =(. There is a regulator that makes it capable of being <em>powered</em> by 12 volts, but the input pins are different.</p>

<p>If you look at table 30-1 on the datasheet for the Uno's controller <a href="http://www.atmel.com/images/Atmel-8271-8-bit-AVR-Microcontroller-ATmega48A-48PA-88A-88PA-168A-168PA-328-328P_datasheet_Complete.pdf" rel="nofollow">here</a> you can see the input characteristics. The arduino is powered at 5v, so the max digital input voltage is 5.5v</p>
