a while ago means some time ago. The period of time we're talking about can be a day, a week or even a couple of months. It is always relative to the situation in which this expression is used.
Example #1:
— When's the last time you saw Mike?
— Actually, it was quite a while ago.
Here, it probably means a long time ago.
Example #2:
— Is the boss in?
— He was here a while ago, but I don't know where he is now. Maybe, he went to lunch.
In this example, a while ago probably means a couple of hours ago.