As dave says, let us know the code.
The O2 sensor codes are the most common occurrence and 99.9% of the time nothing to worry about. They usually happen on a cold day and go once reset.
If it persists, then probably worth looking into, but wouldn't worry for now. It can just be a bit annoying having to reset it .
You can go on eBay and buy yourself a OBD2 reader for about £20 check the codes and reset yourself. You can even get Bluetooth ones and an app on your phone to do the same. Also comes with lots of other toys to play with.
If you look in the guides section for ecu reset or pedal dance and have a read you can read and reset your fault codes without any equipment.