Is it theft to take my motorcycle from mechanic if he has done no work on it?

Yes, there are legal ramifications. More information would be needed to determine if he would be in more trouble than you.

I agree with the previous posters that are telling you to get a police officer to accompany you to the garage.  Make sure you have any relevant paperwork with you such as your title, bill of sale, etc.  

You might have to trespass in order to get to the bike. (Barging past an "employees only in this area" sign is trespassing.)  But, if you call the sheriff first and ask them to stand by while you get your bike. They won't help you get the bike, but it would impress the mechanic.

It is your motorcycle. It is never theft for you to take your own property. You owe him for any work done, but that is separate. He cannot withhold your property, period, even if you owe him money.

I do not see why not if it is titled to you. You could even contact law enforcement if you want to make sure there is no trouble.

Its your property, you can go to his buisiness and collect it any time. He doesnt hold any rights to keep your propery from you, short of you owing for any work that was done on it