I was playing around with a project last night and uploaded once without disconnecting my LCD-00813 from TX. It, naturally, did it garbling thing, but now my LCD is very dim. Even after several resets.
I include both Clear and Backlight full command in Setup.
//Clear:
Serial.print(0xFE, BYTE); //command flag
Serial.print(0x01, BYTE); //clear command.
//Backlight full:
Serial.print(0x7C, BYTE); //command flag for backlight stuff
Serial.print(157, BYTE); //light level.
Sounds like you may have inadvertantly set a brightness setting into the LCD's memory. Is it a Serial Device? If so there are commands to clear the settings back to default. If not you will need to read the datasheet and figure outsome kind of default setting command.