Unless you OVER volted, I don't think there is any way you could have damaged the machine irreversably.
Try holding down the F8 key as you boot up.
If you still have no luck, call Acer and tell them that your computer failed (you may not want to mention any experiments you did on Undervolting etc). That assumes that you are still under warranty. If not... does the pc beep when you boot it ? If not... the CPU is probably either fried or loose. If it does beep, the beep pattern should tell you what the problem is (you know your post beeps off by heart, don't you ? If not, look at www.biosbeeps.com