welcome to the forum. it sounds like you could have been hit by a malware.
scan with malwarebytes antimalware.
http://www.filehippo.com/download_malwarebytes_anti_malware/download install update and do a scan.
you could also do a scan with avast and see if it picks up anything.
if we need to give you more support we would be glad to know a little more information on your system.
like whats your os and what version of avast you using?
good luc and let us know on the progress.