Unless you have a buddy who's a bike mechanic, you'll probably have to take the trike and the broken chain to your local bike shop. There's a thingy called a power link in there that connects the two ends of the chain together. If that thing just came unhooked it can be put back together. If the chain truly broke, there's a simple tool that will press the remnants of the broken link out, and you can replace the broken link with another of those power links. The thing is, if your chain really broke, the next weakest link might break soon. None of this is rocket science, but some expert help is what you need. A knowledgable mechanic can inspect your chain and tell if it needs replacing or if it should be repaired. Having the shop replace your chain will likely cost you between $50 and $100, less if all it needs is cleaned and repaired. Good luck!