Whipped cream is essentially aerated fat (sorry), and the best way to hold the bubbles and air into a fat is to keep it cold. Think about frozen ice cream versus melted ice cream. If your cream won't whip, try placing the bowl and beaters in the freezer for 10 minutes, and trying again. On the flip side, if you've over whipped your cream and it's a chunky mess that looks more like butter, drizzle in a bit more cold heavy cream and beat until fluffy again.