I think my durso pipe lost its syphon

[twiddledog]

I did just a water change and turned the power off so my overflow did its job and filled my sump which is fine. However when I turn the power back on the sump pump is working and pumping water to my tank return but the overflow is filling and not equalling out. The sump is being emptied with no water returning back down to the sump.
Could it be clogged? I have tried to figure it out and I can't. It has worked flawlessly for years until now. Nothing has changed.
Anyone have any ideas?

 

[S Hunter]

Check for a clog down the pipe and make sure the vent it self isn't clogged..