iphone 4, broken top button, will not power on.... just a blank screen.... any help?

If your top button isn't working; you should consider replacing it. A hardware failure like that would prevent the ability to power off the device/hard reset it....THe best you can try to do is a forced recovery mode if able

http://support.apple.com/kb/HT1808

If not; contact apple to see what your support options for replacement are

1. Charge with AC adaptor for at least 30 minutes

2. Reset the phone (no data loss): press both the home and power (top) buttons for up to 1 minute, releasing as soon as the Apple logo appears

However if the power button is actually broken, nothing you can do other than getting the phone replaced or repaired.

Out of Warranty replacement cost for the iPhone 4 is US$149. They don't fix the sleep/wake button, they replace the device with a refurbished.

Apple doesn't repair iPhones currently. They will give you an out-of warranty replacement, usually a refurbished phone, price depending on your country.