I have recently look on the Avast Free Antivirus interface and I have seen that all my File, Web and Mail shields are off and will not turn on. I am using the version 17.6.2310 (build 17.6.3625.0). Can anyone help me?
Do as has been explained in many other threads and see if it solves the problem.
Repair avast and check if the problem is solved.
If not perform a clean installation and check again.
Please (re)search before posting as things like this haven been told many times already.
Don’t let Eddys reply discourage you. 
Here’s the information you need
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
If you need additional help with the Clean Install, watch this:
https://youtu.be/p-h3myRD51A