Have you tried to pull the battery from your phone and then try to put it back in and try it? Normally that fixes any issues like that. Also make sure that your phone is showing 3G enabled in your settings app.
Maybe try doing a master reset. You will have to redownload all of your apps, and will lose all your pictures and songs so I would back those up to your computer first. Hope that will work for you and just let us know if it doesn't.
If you haven't been able to get the the Ace to factory reset, you can reset the device using the Android System Recovery. The video has no sound, the instructions are in the description section of the video. Link below.