Why my car is running rough on start up and running hot.

What is your question?

Started my car and it's running rough. Was coming from the store around the corner and the check engine light started flashing continuously. Thank goodness I wasn't to far from home and made it okay. Opened the hood and the coolant reservoir was boiling hot but the temp gauge was showing normal. This was my first encounter of this problem with my car. Hearing whispers my Cataylic Convertor could be clogged/stopped up. If so, what would be a ball park figure on the repairs so I can brace myself. Any and all info is greatly appreciated!! Have a blessed day to all!!