I replaced my alternator on my 2011 Toyota sienna and now my car won't start.

So, the engine cranks but will not start? Check all fuses and relays to make sure nothing has been disturbed or blown. Ensure a proper key that is programmed to the vehicle is being used. Following this, testing utilizing a diagnostic tool for engine performance will be required. What was the original concern where the alternator was seen to be needed for replacement?

Was the battery disconnected during alternator replacement?