Check that you haven't got any while loops running without a thread sleep.
Code:
// This will eat all your cpu
while (true)
{
// Do something
}
// This will not eat your cpu
while (true)
{
// Do something
System.Threading.Thread.Sleep(1);
}
It's one thing I have forgot before and the cpu raised to 100% :P
If it's not that then have you changed anything? If so, what and have it always done it?
We need some more information to help you.