welcome to the forum daphnep.
its sounds like the file its freezing on as you say could be a large file so it takes some time to scan it.
but to be able to help you better we need a little more information.
what os you using?
and what version of avast you using?
did the quick scan come up with something?
i suggest you let it run for and see if it takes more time to scan the file or its just stop scanning at that file. if so it could be a bug, but i think it sounds like it’s taking more time to scan the file.
hope this will help you
good luck and let us know on the progress or if you need more support.