Insurance. The bike might be covered by homeowners' or renters' insurance, and individual policies are also available for bicycles. Make certain the policy covers replacement for a totaled/stolen bike; if insurers can wiggle out of paying, they will.

Document your ownership. Snap clear pics of any receipts or other paperwork demonstrating your purchase and file the originals. Register it with the city you're living in. Also have some pics of yourself traveling with the bike. Goes a long way toward favorable treatment from otherwise unsympathetic authorities.

Call a friend and tell them you'll reimburse them for some bolt cutters and/or a hacksaw. Don't leave the bike unattended for more than a quick bathroom break, certainly not past dusk. Take pictures/video of the problem, both at the moment you find the unwelcome lock and before, during, and after the process of removing it yourself. File a report with the authorities if possible, even if they don't take immediate action, it will go into their system and they might catch the offenders later.

Keep in mind, these thieves/pranksters didn't want a confrontation. They'd have broken the lock (or the bike rack) in broad daylight if they wanted to be that brazen. As long as you are near the bike and there are a few witnesses roaming around, they have only cost you time. Even so, if it comes down to it, don't linger in a dangerous neighborhood too long after dark -- your safety is more important than your bicycle.