You don't say what avast version number you are using, nor the operating system you have avast installed on ?
Both of these have an impact on what to do.
The instructions below assume you are able to run avast 19.x.xxxx and are using Windows 7 or later.
Have you tried a clean reinstall, utilising the avast uninstall utility ?
If not try this:
Download the latest version of avast, see
https://forum.avast.com/index.php?topic=228911.0 and save the full off-line installation file for your avast program to your HDD, somewhere you can find it again (if you didn't save your last download). Use that when you reinstall.
- Download the avast! Uninstall Utility,
https://www.avast.com/uninstall-utility avastclear.exe and save it to your HDD.
- 1. Now uninstall avast! - using add remove programs/Programs & Features, if you can't do that start from the next step, reboot.
- 2. run the avast! Uninstall Utility, it will request it be run from safe mode (and will handle the boot into safe mode), once complete reboot into normal mode.
- 3. install the latest version, reboot.
Note: save the copy of avastclear.exe so it can be reused as some users are reporting that after running it, it is self-deleting.