Avast won't start - help!

Out of the blue my computer told me Avast needs to reboot (Win7, don’t know Avast version)

So I reboot, now Avast opening screen just says ‘Loading…this shouldn’t take long’ and the interface shows I have no virus cover

What is going on?

First of all try to repair the antivirus.
https://support.avast.com/en-ae/article/Repair-Antivirus
Then, if that didn’t work, uninstall and reinstall Avast.

If these solutions didn’t work, please give me a reply.

I tried the repair but it made no difference, will now try ununstall and reintstall

A complete uninstall and reinstall seems to have worked so far, but I still have no idea what happened. a full virus scan showed nothing

Sometimes Avast does an emergency update along with a virus definitions update which requires a restart to take effect. I can only guess that must may have been what you encountered and got corrupted. If you have now done a clean install and a full virus scan without issues, then you should be good to go.