Several problems can be associated with this problem
If running heat it is likely a drain hose or Drain connection that is plugged up. Simply blowing these lines out can solve your problem.
If running AC it is likely a Drain from the coil to the Drain line that is plugged up, or you have frozen the unit up. Check the unit outside, if the big copper line or compressor is frozen, turn system off immediately and call a service company to check the system out.
I also recommend changing your air filter every 3 months if you haven't done so.
Answers & Comments
Several problems can be associated with this problem
If running heat it is likely a drain hose or Drain connection that is plugged up. Simply blowing these lines out can solve your problem.
If running AC it is likely a Drain from the coil to the Drain line that is plugged up, or you have frozen the unit up. Check the unit outside, if the big copper line or compressor is frozen, turn system off immediately and call a service company to check the system out.
I also recommend changing your air filter every 3 months if you haven't done so.