I have a Roca dual-flush toilet but it isn't flushing at all?

There is no water sound at all either so I think I need to remove the top off the cistern to check if there is any water inside - unless someone else can tell me otherwise?
We have running water in the flat and there is still water in the wc itself but I'm having to throw a bucket of water down to flush?? This is a relatively new toilet but this is now the second time this has happened - the first time is rectified itself after about an hour - this time is has been overnight and still nothing?? Please help.