I know it may be a little late now since you're question was a couple months ago, but I have had this problem with some 9090's before too where a warm boot (holding the power button down) will not bring them up, but a cold boot always does (unless of course it truly is defective - that's not usually the case though).
To do a cold boot, pop the battery out part of the way until it "catches" (you'll know what I mean if you have an MC9090 and try it). Then hold down the trigger and the power button simultaneously while pushing in the battery all the way. Once the battery has been pushed in, release the power button and the trigger.
This should cause the MC9090 to boot up fine. Make sure you release the trigger after doing the steps above. Sometimes people don't do that, and this causes the MC9090 to go into an infinite warm boot where it just continuously reboots over and over. The only way to get out of it is to cold boot again, but this time release the trigger in time.
Thanks,
Brad Radaker
Radicomm (makers of QuickTalk - push-to-talk over WLAN for the MC9090)
www.radicomm.com
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