If you have a valid disabled parking permit, you can request dismissal of the handicap parking ticket. Otherwise, you'll need to prove that the handicap sign was not visible to a reasonable driver, or the space you parked in was not a handicap spot.
You thought it was no big deal to park in a handicapped spot while you popped into the dry cleaners for two minutes. Only now, you have a traffic ticket. And of all the possible parking violations, this is one of the most serious. Fines for parking in handicapped spots without a permit are high dollar in most jurisdictions, typically in the $250 to $450 range, and you'll also get points on your license. If you are caught parking in a disabled spot and think you are not at fault, you can fight the ticket. You'll need a legitimate defense, however – the fact that there were no other spaces is no excuse.
TL;DR (Too Long; Didn't Read)
If you have a valid disabled parking permit, request dismissal of the handicapped parking ticket. Otherwise, you'll need to prove that the handicapped only sign was not visible or the space you parked in was not a handicap spot.
If You Have a Disabled Permit
If you have a valid disabled parking permit, you can usually request dismissal of the handicap parking ticket by filling out the Affidavit of Compliance provided with the citation and mailing it to county parking enforcement. You'll find the address on the citation. You're also permitted to use a handicap spot while transporting a disabled person. If that's what happened, fill out the Affidavit with details of your passenger's permit.
If You Didn't See the Sign
States and municipalities have specific rules about how handicap parking spaces must be marked. While rules vary between jurisdictions, you'll typically find that a space has to be marked by blue or yellow paint on the curb with signage consisting of a profile view of a wheelchair. The key point is, this signage must be adequate and visible. It's not your fault if signage is missing, damaged, overgrown with shrubbery or situated in a location that no reasonable driver could see from the entrance to the parking space. Enter a plea of not guilty if you didn't see the signage, then take pictures of the situation so you can argue your case in front of a judge.
If You Were Not In a Handicap Spot
If you received a handicap parking ticket but were not in a handicap spot, you were not parked illegally. Contest the citation, and the case should be dismissed. You'll need photos of the space to show the ticket was wrongfully issued. Another common scenario is that in which you were using the handicap space to turn or move your vehicle. Contest the ticket if you were still in the car, since a parking violation implies that the vehicle was turned off and unattended.
If the Handicap Space is Redundant
It's possible to fight a handicap ticket based on where the space is located. Generally, a handicap parking space should be located close to the building it's intended for. If that building no longer exists, or the other handicap spaces have been moved, you may be able to show that the space where you parked should not have been designated for handicap parking in the first place. This defense is a bit of a long shot, and you likely will need a traffic attorney to help you argue the case.