I just had the a similar issue, running Retropie 4.0-beta2.
After rebooting, I got stuck to the "No gamepad detected" screen. I'm using a ps3 bluetooth controller, I see this every time I boot, while the controller is connecting to the pi, after which the main emulation station screen usually show up. That didn't hapen today.
I had a look at the emulationstation log file (~/.emulationstation/es_log.txt) and found the same error as you:
lvl1: requested mismatched theme type for [detailed.md_releasedate] - expected "datetime", got "text"
As well as a line confirming that my ps3 controller had in fact successfully connected , which lead me to think that I was stuck on the "No gamepad detected" for some other reason than no gamepad being detected:
lvl2: Added known joystick PLAYSTATION(R)3 Controller (instance ID: 0, device index: 0
I fixed this by running Manage packages > Update all installed packages in the setup script (using SSH), but simply re-installing the carbon theme in /etc/emulationstation/themes/carbon should do the trick.
Hope this helps!