FAQ: RED LED of death - ruuvi/ruuvitag_fw GitHub Wiki
There have been cases where after updating the firmware the ruuvi returns to the DFU (Device Firmware Upgrade) and does not enter the actual application.
The ruuvi development board along with the Nordic development kit and nrjprog can be used to recover the device.
nrfjprog --family nrf52 --eraseall
nrfjprog --family nrf52 --program ./builds/hex/sdk12/ruuvitag_b3_debug_bootloader.hex
nrfjprog --program ./nRF5_SDK_12.3.0_d7731ad/components/softdevice/s132/hex/s132_nrf52_3.0.0_softdevice.hex
nrfjprog --family nrf52 \ --sectoranduicrerase \ --program ruuvi_examples/ruuvi_firmware/ruuvitag_b/s132/armgcc/_build/ruuvi_firmware.hex \ --reset