It doesn’t ignore for all time, just that instance, it will be scanned the next scan you do, but avast won’t let a file run that it considers infected. So you are still relatively safe.
Run a scan again and it will be detected again, then select move to chest. If you know the file name all you need do is right click on the file name and select the Scan selected areas option.
What is the infected file name, where was it found e.g. (C:\windows\system32\infected-file-name.xxx) ?
Check the avast! Log Viewer (right click the avast ‘a’ icon), Warning section, this contains information on all avast detections. C:\Program Files\Alwil Software\Avast4\ashLogV.exe
Or check the source file using notepad C:\Program Files\Alwil Software\Avast4\DATA\log\Warning.log and copy and paste the entry.