Are you sure you claimed a residence in the first place? (Type /v open, then /res claim [res number])
After 10 days you residence goes on the derelict list. Meaning it can be reset if anyone asks for it. After 30 it's supposed to be reset etc. as well. This could have happened.
If you had claimed a res in the past but weren't on for 10 or more days, then somebody may have requested to have your res reset so they could claim it. (For more info, go here: http://empireminecraft.com/guide/derelict)