So I am a bit unclear still, were you able to boot into recovery (by turning on the evo by holding volume down and the power button)?
Once you are in recovery we can do a few things from there to help - many of which are listed above my post. It would help if you could provide the information listed above as well. So here are the things to tell us.
1. Can you get into recovery? (volume down and power button)
2. Did you make a nandroid backup before flashing cm7?
3. Did you follow the install instructions EXACTLY for installing cm7? Meaning did you do a complete data wipe, cache wipe, dalvik wipe, etc.
Once we have those answers we can take it from there.