Board not booting after using PCIE 1x slot

I've purchased a set of the following:

- LattePanda Mu (DFR1146)

- Full-Function Evaluation Carrier Board for LattePanda Mu Compute Module (DFR1141)
- Aluminum Active Cooler for LattePanda Mu Compute Module (FIT0981)
- 19V 90W AC/DC Adapter for LattePanda Mu Full-Function EVA Carrier Board (FIT0988)
 

After initial setup and installation of Ubuntu Server (which went well), I powered off the board to install an PCIe riser containing an NVMe drive in the PCIe 1x slot. However since then the board will not power on anymore even after removing the riser. There is no power indication (blue light on the button), the fan on the heatsink does not spin and the system does not POST. 

The only things connected to the board are the power barrel, HDMI cable for output, a keyboard and a WiFi dongle on USB.

System simply does not turn on.

Anyone have any suggestions to get this working again?