I made a mistake when drawing an image to the screen: the width/.height parameters got arbitrary values. The screen went black and since then it won't synchronize and start its selftest during power on. A small portion of the screen flickers white and then it goes all black. Could my mistake have caused random data to be written to the firmware?

If there was no flash chip on the board, then the welcome screen and user fonts will be stored in the MCU, so, in some case when writing welcome screen or user fonts with wrong parameters, the firmware in MCU might be erased partially, but can be retored by user self to upgrade the firmware:

How to: https://www.digole.com/forum.php?topicID=653

Firmware Download: https://www.digole.com/fw/