There should be a release valve on the top of the filter that shuts off the two hoses and allows you to disconnect them. Once these are disconnected you can take the main part of the filter to a sink and open it up to clean it out.
It sounds like you didn't shut off the hoses before you opened it last time, so water started to syphon out of your tank.
See on this picture where the hoses connect from the top? Notice that lever inbetween them? This is the flow control and hose cut off. Pull that upwards through 90 degrees (gently) so it closes off the hoses. There is then a release mechanism underneath that lever to take the hoses out. When putting them back on, just do the reverse.
Oh, also when you clean the filter media, clean it in water taken from your tank. I tend to put the sponges into the water and scrunch them about to get debris out of them. Likewise for the biological media (ceramic hoop things probably). Give them a swill about in the water to get any bits off them.
**Don't rinse biological media under the tap** (You can rinse the sponges if needed). The chlorine in the tap water will kill of the bacteria and you'll have to cycle your tank again and this will cause problems.
Also, when doing this, try to keep the biological media in tank water all the time.
Appologies if you know all this but best to cover all the bases
