Canister filter damage inspection, check these things
Now, the most likely problem is that when you primed the unit there was some air left in the unit and during a brief power outage that air drifted back up the input line, breaking siphon. Then the unit tried to come on again, if there is a heat shut off like the Eheim, it has shut down to protect itself.
A leak on the intake line will do that, also, and if the impeller well cover is off it cannot pump. So, check the hose connections on the inlet and outlet first -- an inlet leak will allow air in, an outlet leak will spray water. Then check the impeller cover.
Since I just spend a long time on the phone with Hawaiian Marine getting help on the Eheim I thought I'd destroyed, I can walk you through some things to check. I'll just assume that most of these things will be similar.
So, first, check the hoses. No, wait, first be sure the unit actually has power, is it plugged in and any power directors switches on, any GFI blown in the circut? Be sure the outlet has power!
Now, check the hoses. A bubble on the inlet line tells you that you got air in somehow. Any airstones nearby? If not, then the hose may be leaking somewhere on that inlet line, check all connections. check your valves -- any left closed (been there, done that, Eheim 2213 survived that for 3 hours)
If not found, proceed to open up the canister. Check to see the impeller well cover is closed. If that looks good, remove it and remove the impeller to check for damage. Put some metal thing like a straightened paperclip into the impeller well and turn the unit on, if it vibrates it is getting power. Inspect the impeller for cracks or serious looking marks. (If the rubber bushing stays in the bottom of the well, don't worry about it, you will have to remove it to replace the impellor or shaft but it is Ok to leave it there for now).
Reassemble the pump head and run about 2" of water into the sink, put the pump head in it and turn it on -- be prepared to deflect water back into the sink if it works.
That seems to be all there is to it. You've tested the power line from the wall to the impeller well and looked at the impeller itself, tested the flow in the sink. You've looked at the lines, the shutoffs and the connections.
Just a postcript to my own filter troubles, I managed to put the Pro2Thermofilter back together out of parts from the other Pro2, switching heads and impeller covers and gaskets since the Thermo had a bad head and good cover and gasket, the other had a bad cover and bad gasket on a good head. So, now I have just one unit out of service needing an impeller well cover which I've ordered and maybe a gasket, which I already have on hand I think and that may actually fix the problem, or maybe not -- I'm still not certain that the head is bad or if it was just on thermal shutdown. A $4 part will tell me if I need to order a $80 head (unavailable) or a full replacement unit (and kick myself around the block for messing it up)
Meanwhile I've also ordered an impeller well cover for the Fluval 303 which will put it back into service.
Well just the hoses.. heh