Firstly, try to contact them again to get confirmation (via your MindMyHouse Message Centre). It may that they are delayed, or unable to access the internet. It has been known to happen!
If you want to cancel your house sitter and choose another one, first you need to 'unselect' your sitter. See this FAQ for detailed instructions on how to do this. Once you've unselected them, you can then go about choosing another house sitter. You can also contact us at the helpesk, in case we can assist in trying to contact your house sitter.