If you took your only pill yesterday, you really just have to wait. Most people who get side effects have it go away so your chances are good still. We don’t really have good enough data to say that of the guys who develop side effects, 75 or 90% see them resolve. It’s most definitely most though.
You can’t conclude you have PFS for awhile. As a rule of thumb, we generally say that you need to have side effects continue for 3 months before you consider that you have PFS. Once you are at that point, it’s likely you do. I’ve only seen a few guys say it took them 6-12 months to recover after developing side effects, but I have seen it. Maybe their problems were unrelated to Propecia.
For our sake, please keep yourself calm for the time being. You have a good chance of getting better and it makes our community look bad when people get side effects after one pill and start freaking out about it and making a lot of noise all over the internet.