My friends took my PC to a repair shop without my approval can the repair man legally hold my PC?

My roommates took my PC to a shop they say for adding ICONS and security software. They did not ask my permission and it was in violation of the contract we signed, I created a contract with them that that they could have the PC in exchange for a 200 credit on rent. I also had a clause that i was to do any service on it. When i write contracts i always add a loophole so if i decide i want to back out I have a way to do it. Can the PC repairman legally hold the computer if they took it to him without my authorization. My understanding is that if they reneged on a contract my bringing it to a repairman outside that him holding would constitute stealing. I know everything that was installed on it including the serial number and i have credit card statement from where it was bought originally