What happens to you when you ignore a jury duty summons depends on what state you're in and what the laws in that state are regarding ignoring the jury duty summons.
Some states are very serious about people missing jury duty summons while some other states are not as serious about it.
In some states if you ignore a jury duty summons they'll fine you from $250.00 to $2,000.00 and some states will put a warrant out for your arrest and send a police officer to your door to find you and take you to jail for at least 3 days.
So you would have to check with your states laws regarding the actual action that would be taken in your state.
For me personally nothing has ever happened to me when missing jury duty or ignoring the jury duty summons and I've always just thrown the jury duty summons away.