The majority of problems are water-quality related. Have you changed the water since you've been back home?
We need to know a little more about your setup really.
Tank size/water volume? How often do you change the water and how much? When did you last water change?
Do you use a water conditioner?
Is this a cycled or uncycled setup? (is there a functioning biological filter or do you just water change alone?) What is the name and size of any filter
Test Results for the Following if you have a filter:
Ammonia Level?
Nitrite Level?
Nitrate level?
Ph Level?
Any medications added to the tank recently?
What do you feed your fish?
A picture would be helpful too