How cn I fix my closed cistern push botton flush toilet?

The internal mechanism of our toilet cistern has come undone and I can't figure out how to reconnect it so it will flush. The push button (whole single button, not split or double) is in the non-depressed position but nothing is holding the water re-fill part up so it's constantly running water into the bowl unless I wedge it up. Any suggestions on how I can fix this myself or do you think I need a professional? It came with the house and I can't identify the brand or model.