"'Can I file a mechanics [lien] even though I don't have the car anymore?'
Here's the form:"
Jack, that form is the certification by the repair shop that the requirements to SELL the car pursuant to the lien have been met. It is not a form to obtain a lien on the car. While the form calls it a mechanic's lien, that is not actually a correct term for the lien. A mechanic's lien generally applies to real estate. The lien auto repair shops have is usually called a labor or repairman's lien. Typically these liens only allow the person who did the repair to hold the car until he is paid and then, after some period of time (often 30 days) to sell the car to recover the money owed following procedures for the sale set by the state. These liens only last while the car or other repaired property is in the possession of the person repairing the property. If the repair person gives the property back to the customer, as here, the lien generally terminates. At that point, all the repair shop can do is sue for a money judgment to get what is owed. My search for Illinois law suggests that this generally how it works there, too.
"Winning is a slam dunk."
Any lawyer with any amount of trial experience will tell you that winning in court is never a "slam dunk." The poster might have a very good chance of winning, but we don't have all the facts, either. IL law apparently requires auto repair shops to provide various notices and conform to certain rules. If the shop did not do that, that might form a defense or allow some sort of counterclaim.
The poster should consult an attorney to find out what to do to recover his money in this case, and to find out how to protect himself better in the future.