The engine would stop running. I had to guess at which engine you have so I picked the 2.4L. That is an "interference" engine meaning the open valves will be hit by the pistons as they coast to a stop, and will be bent. You don't have the tools to replace the bent valves, and to rent or buy them is not practical if you're only going to use them once. You just need regular hand tools to replace the belt but the valves requires removal of the cylinder head. The head surface must be checked for flatness, and the block and head surfaces must be prepared with the proper surface finish for the new gasket to bite into and seal properly. Cylinder head bolts must be torqued properly to insure sealing and to prevent warping the head. Improper torquing of the head bolts typically shows up as a leaking head gasket after numerous warmup / cool down cycles. That could take weeks to show up.
It's doubtful you have a bad timing belt at 20,000 miles.