The main thing is why didn't it update again if it was corrupt?
We are not saying that the download was corrupt. The server may have been busy, it could have had something do to with your local Internet traffic, there are many variables.
In situations like this, the first thing to do is to try your download another time, and if necessary force it manually at a later time.
If you still have a problem, then try an Avast Repair:
- Go to Control Panel > Add/Remove programs > Avast Antivirus.
- Scroll down and choose Repair function in the pop-up window.
- Reboot.
If you suspect your machine is acting strange and you have another problem, check for malware. Otherwise, if you think there is a problem with Avast not working properly after doing a Repair and it is unsuccessful after doing a reboot, then you would do an uninstall/clean install as a last resort.
Most times, just waiting for the next Avast definitions update resolves the problem.