Arduino power short, Irrelevant measuring after power short

Hello. I have a serious problem, and you might help me. I Made a simple temp. sensor with an arduino mega 2560, and a LM35 temp. sensor ic from TI, and a ks108 128x64 lcd module. The power supply is a 3cell li-po, that is 12.6V at full charge, but it was ok, none heating.

So, yesterday when i put it to the desk, suddenly the +5V pin of the LM35 touched the outer metal enclosure of the USB connector on the board. It reseted, and worked as before. I don't know why, maybe curiosity, tried to touch the shield(ground) with the lm35's +5V pin again, it rebooted normally, like before.But after the second try, i noticed, that the temp sensor began to measure seriously irridiculous temperatures. although i It is set to measure every 30 sec, and in a 22C room, it firstly measured 15 Celsius, after 30 sec, 22 Celsius, and again 16, and different every measure. And I noticed, that the maximum contrast that can be set via the same potentiometer, gradually increased. It can be even set to light up every pixel. It could not be done before my silly action.

So i think i burned something, but i checked the panel, can't see and smoke, burnt traces, or even smoke, or bad smell. The voltages are OK, i checked.

BUT, if i connect it to USB power, it is normal, works as before the shortage.

So maybe i broke something which is connected only to the external power side, not the usb? Any deas?