Restarts to bsod in a loop.
Safe mode not helping.
Startup repair boots to black screen with a mouse pointer and nothing else.
Can someone get me out of this mess?
You need to provide more info. for anyone to help 
What Avast
What version
Edit:
What other Security software installed/uninstall recently.
Avast free.
No access so cannot verify latest version.
It was updated two days ago.
No other software installed/uninstalled/reinstalled
Last file that showed when booting in safe mode is aswbidhsa.sys
You could Try a Clean Install as it’s possible some code got messed up.
Download:UNINSTALL UTILITY
https://support.avast.com/en-ww/article/Uninstall-Antivirus-Utility
Re:Bob3160
Clean Install of Avast:
https://goo.gl/4Ptzkf
Cannot be done as I have no access to the OS
Avast update rebooted the OS for crash and reboot loop.
Do you have a recovery volume, rescue disc available or the original OS install discs?
I did a quick Google search and this is not a new problem, I quickly found several threads from last year and this year reporting the same thing here:-
https://forum.avast.com/index.php?topic=208626.0
https://forum.avast.com/index.php?topic=211701.0
https://forum.avast.com/index.php?topic=214090.0
There are also a whole load more threads on other forums but the problem is that the cause is never reliably identified and the ‘solutions’, when they work, seem similarly random. The blame when not centred on AVAST is put on HDD failure, graphics card driver updates, MB problems/failure, boot mode, BIOS corruption and the usual lazy blame all: viruses/malware of course.
The file it is hanging on: aswbidsha.sys is described as being an AVAST Application Activity Monitor Helper Driver and the most plausible suggestion, based on the main evidence: this occurs most commonly after an AVAST update, is that it has been corrupted or damaged in the process.
What is apparently the AVG equivalent: avgidsha.sys was also being cited in a least one identical sounding case I found over three years ago.
Not being able to get at AVAST to uninstall it, or use a restore point to roll it back, because the fault means all safe modes are unavailable IS the problem.
Unfortunately if there is no way of doing this and none of the other solutions suggested work the last resort advice is to use a rescue disc/volume, repair tools or, worst case, a reinstall of the OS. 
Usually, the last file listed isn’t the problem but the file right after that in the boot order.
The way to recover however doesn’t change.
Identifying the particular problem file, as it was identified in the same way in all those past examples, would still likely be the same one wouldn’t it ie. the one after awsbidsha.sys?
But as you say the way to recover is the same whatever the case.