Well gobylover,
Could be a lot of things.
What type skimmer is it ?
Venturi ?
Airstone ?
If venturi type - If the powerhead is running and no bubbles - then most likely a clogged venturi inlet tube ( hose )
If airstone - probably time to get a new stone - could be clogged as well.
My guess - Something is CLOGGED UP !
More info por favor