Bootloop after flashing TWRP (Also lost recovery) So I was going to root this (previsouly healthy) BQ A4.5. Unlocked the bootloader and flashed TWRP from here:

$ fastboot oem unlock
(bootloader) Start unlock flow

OKAY [ 32.954s]
Finished. Total time: 32.960s
$ fastboot flash recovery downloads/twrp-3.3.1-0-dendeone.img
target reported max download size of 134217728 bytes
Sending 'recovery' (11342 KB)...
OKAY [  0.378s]
Writing 'recovery'...
OKAY [  0.335s]
Finished. Total time: 0.727s
$ fastboot reboot bootloader
rebooting into bootloader
OKAY [  0.003s]
Finished. Total time: 0.058s
$ fastboot reboot
Rebooting...

Finished. Total time: 0.058s


The first reboot to bootloader didn't do anything.
Now it's on a bootloop which states:

Orange State
Your device has been unlocked and can't be trusted
Your device will boot in 5 seconds


Pressing Volume Up + PowerOn used to work for recovery (that's how I got into fastboot mode).
Now, pressing Volume Up/Down/Up+Down + PowerOn doesn't do anything, fastboot stands indefinitely &lt; waiting for any device &gt;, and adb always responds with error: no devices/emulators found regardless of when it's run.
If I press Volume Up+Down it turns off. Plugging in USB it turns on. Nothing else seems to respond.

dmesg states:

[32774.364937] usb 2-7: new high-speed USB device number 59 using xhci_hcd
[32774.514434] cdc_acm 2-7:1.0: Zero length descriptor references
[32774.514450] cdc_acm: probe of 2-7:1.0 failed with error -22
[32774.554519] cdc_acm 2-7:1.1: ttyACM0: USB ACM device
[32777.142205] usb 2-7: USB disconnect, device number 59


The disconnect happens before the bq logo appears.

Trash bin time?
Same problem with my BQ M8... 
Here is how I solved it:

-With the phone conected via USB, Launch CMD to run ADB and test as many times as needed that the PC and terminal can talk to each other with the command "adb devices". Eventually after several attempts, it will return the device and number. Posibly stating the work "recovery".

-Go into bootloader with the command "adb reboot bootloader". As before, try it as many times as needed util it works (that means you will have to re-Run the ADB, and un-plug, plug the phone).
-Once on Bootloader (you can verify because the phone screen is in "Fastboot..." state, you have to Launch in your PC the FLASH TOOLS (hpoe you know how to use it)
-In FlashTools select only BOOT and RECOVERY tickmarks, and leave the rest unmarked). Then Press on the DOWNLOAD green arrow
-Now back in ADB window, use the command "fastboot reboot". This will re-start the phone, and FlashTolls will immediately kick in, Flashing the BOOT and RECOVERY files back in your phone (it takes miliseconds).
-The phone will re-start and this time you will have all in STOCK (you´ll also have Bootloader Locked again).

Hope this will help you out.