I've had the same problems as OP; I would open Tequila and it would go through the downloading phase and never reach done, it would just close. I got tired of restarting my computer when it happened because it was only intermittent, and not a regular occurrence.
Since you said you have AVAST, try looking in the virus chest. I found out today that my AVAST was putting 1 small file in there, which forced the application to not work properly. A simple restore and exempt got it back to working like it used to.
Hope this helps, and if it doesn't all I can suggest is to go here .
Read through everything, don't just skim down to what you suspect to be the issue.
