The deal is that up to yesterday my Leonardo-clone was behaving perfectly, with the assistance of a DHT11 and a two-colour OLED advising me of the temperature and humidity in my room.
I wanted to add another sensor and spent some time messing around with wires and pins (I'm pretty confident I didn't wire-up things in a damaging way) but couldn't get it to work as the Arduino IDE couldn't find the serial port (previously COM7). I've tried different USB cables (which I knew "worked" - the IDE sees my 'ordinary' Arduino on COM10) and connected to a PC-USB rather than through a USB-hub but without success: when only the Leonardo is connected "Port" under "Tools" is greyed-out in the IDE. So when I try to run "blink" I get red messages about Couldn't find board on selected port (of course there is no selected port ...).
On connecting and disconnecting the Leonardo the PC generates the usual USB-device connected bippedy-boo sound (Windows 10) and the Leonardo's green ON and yellow L LEDs come on. If I press reset the L LED waxes and wanes for a few seconds then goes off and the yellow RX LED comes on and stays on.
I haven't been able to find anything on the forum nor via web-wide searching - I really wish it wasn't called Leonardo as I spend a lot of time clicking on links which are about a Mr Da Vinci.