Nano Cube tank help PLEASE!!!

[roobycube]

I'm currently house-sitting for a friend of mine who has a (I'm guessing on the size here...) a 30L Nano Cube Saltwater tank. Today when I got home from work, the water is completely still, and I'm thinking the filter is clogged or something?
If anyone has some advice, please let me know. I'm absolutely clueless when it comes to these tanks.

[OniTweak]

Is the power cord for the power head plugged in? That would be the first thing I would check, not trying to insult your intelligence but it can happen. If that isn't the case then you can take the filter out of the back of the tank and just lightly rinse it to get off debris. If that isn't it then you will have to get your hands dirty and dig out the bioballs and get the trash out of the bottom of the middle compartment.

[stevekellner]

Check whether the power is connected. Take out the filter and clean it.

Any other visible issues.