For the 2nd night running, the Avast icon, at the right of my taskbar, shows an ‘x’ mark through it. When I click on this, I’m prompted to resolve the issue, which I do! Everything appears fine and I’m fully protected. Question is, what could be causing this? I have the free version of Avast which is up to date. Nothing has changed except for the fact it’s the icon which appears to disable itself. Any ideas? Thanks.
Please try running a repair.
Some system details and Avast version could help in future 
if you’re on an old version then update to the latest.
Repair Avast:
Control Panel> Program and Features (Add/remove program)>Select Avast> Select Repair. Reboot when completed
https://www.avast.com/faq.php?article=AVKB204
If Repair doesn’t fix the problem, try the following:
Clean Install of Avast:
https://goo.gl/4Ptzkf